EATING dry ice!


William J. Beaty 3/1995

WARNING: This file is currently being written, edited, corrected, etc. It does still contain some mistakes of its own. I placed it online as a sort of 'trial by fire' in order to hear readers' responses so I could target weak or unclear sections for improvement. (And, as my site points out, NOBODY is perfect so we should always practice critical thinking. Take all information with a grain of salt, including everything here!) Please feel free to send public comments to me with the COMMENT BOOK. If you prefer that nobody else sees your comments, send private comments to me via this form.
          What color is water?

See also another misconceptions list by Dr. J. L. Hubiz

"Lest you think that I am quibbling over minor points of language, I note that in my experience many of the misconceptions people harbor have their origins in imprecise language... Precise language is needed in science, not to please pedants but to avoid absorbing nonsense that will take years, if ever, to purge from our minds." - Dr. Craig F. Bohren, Physicist


also: Electricity Misconceptions      Static Electric Misconceptions

"Errors, like straws, upon the surface flow; He who would search for pearls must dive below." - John Dryden
also: Electricity Misconceptions      Static Electric Misconceptions

That's the way all the books were: They said things that were useless, mixed-up, ambiguous, confusing, and partially incorrect. How anybody can learn science from these books, I don't know, because it's not science. - RP Feynman, in Judging Books By Their Covers

Want books? Try searching amazon.com:

(try "science projects" too)


CORRECT: There is no single list called "The Scientific Method." It is a myth.

See the links and references below.

The rules of a science-fair typically require that students follow THE SCIENTIFIC METHOD, or in other words, hypothesis-experiment-conclusion. The students must propose a hypothesis and test it by experiment. This supposedly is the "Scientific Method" used by all scientists. Supposedly, if you don't follow the rigidly defined "Scientific Method" listed in K-6 textbooks, then you're not doing science. (Some science fairs even ban astronomy and paleontology projects. After all, where's the "experiment" in these?)

Unfortunately this is wrong, and there is no single "Scientific Method" as such. Scientists don't follow a rigid procedure-list called "The Scientific Method" in their daily work. The procedure-list is a myth spread by K-6 texts. It is an extremely widespread myth, and even some scientists have been taken in by it, but this doesn't make it any more real. "The Scientific Method" is part of school and school books, and is not how science in general is done. Real scientists use a large variety of methods (perhaps call them methods of science rather than "The Scientific Method.") Hypothesis / experiment / conclusion is one of these, and it's very important in experimental science such as physics and chemistry, but it's certainly not the only method. It would be a mistake to elevate it above all others. We shouldn't force children to memorize any such procedure list. And we shouldn't use it to exclude certain types of projects from science fairs! If "The Scientific Method" listed in a grade school textbook proves that Astronomy is not a science, then it's the textbook which is wrong, not Astronomy.

"Ask a scientist what he conceives the scientific method to be and he adopts an expression that is at once solemn and shifty-eyed: solemn, because he feels he ought to declare an opinion; shifty-eyed because he is wondering how to conceal the fact that he has no opinion to declare." - Sir Peter Medawar
There are many parts of science that cannot easily be forced into the mold of "hypothesis-experiment-conclusion." Astronomy is not an experimental science, and Paleontologists don't perform Paleontology experiments... so is it not proper Science if you study stars or classify extinct creatures?

Or, if a scientist has a good idea for designing a brand new kind of measurement instrument (e.g. Newton and the reflecting telescope) ...that's certainly "doing science." Humphrey Davy says "Nothing tends so much to the advancement of knowledge as the application of a new instrument." But where is 'The Hypothesis?' Where is 'The Experiment?' The Atomic Force Microscope (STM/AFM) revolutionized science. Yet if a mere science student had actually invented the very first reflector telescope or the very first AFM, wouldn't such a device be rejected from many school science fairs? After all, it's not an experiment, and the list named "The Scientific Method" says nothing about exploratory observation. Some science teachers would reject the discovery of the Tunneling Microscope as science; calling it 'mere engineering.' Yet like the Newtonian reflector, the tunneling microscope is a revolution that opened up an entire new branch of science. Since it's instrument-inventing, not hypothesis-testing, must we exclude it as science? Were the creators of the STM not doing science when they came up with that device? In defining Science, the Nobel Prize committee disagrees with the science teachers and science fair judges. The researchers who created the STM won the 1986 Nobel Prize in physics. I'd say that if someone wins a Nobel Prize in the sciences, it's a good bet that their work qualifies as "science."

Forcing kids to follow a caricature of scientific research distorts science, misleads generations of students, and it really isn't necessary in the first place.

Another example: great discoveries often come about when scientists notice anomalies. They see something inexplicable during past research, and that triggers some new research. Or sometimes they notice something weird out in Nature; something not covered by modern theory. Isaac Asimov said it well:

"The most exciting phrase to hear in science, the one that heralds new discoveries, is not 'Eureka!' (I found it!) but 'That's funny...' "
This suggests that lots of important science comes NOT from proposing hypotheses or even from performing experiments, but instead comes from unguided observation and curiosity-driven exploration: from sniffing about while learning to see what nobody else can see. Scientific discovery comes from something resembling "informed messing around," or unguided play. Yet the "Scientific Method" listed in textbooks says nothing about this. Instead their lists start out with "Form an Hypothesis." As a result, educators treat science as UNplayful, deadly serious business. "Messing around" is sometimes dealt with harshly. See: The Onion, science teacher satire.
"Let me state the Method Position as follows: 'There is something called the scientific method, and someone who understands this method will be able to understand all of science, regardless of the specific subject matter that person has been taught. Thus the goal of science education should be to teach that method.'          It's hard for me to understand how anyone could hold a position that is so clearly untenable. " - Dr. James Trefil,
  "Two Modest Proposals Concerning Scientific Literacy."



"Why should there be the method of science? There is not just one way to build a house, or even to grow tomatoes. We should not expect something as motley as the growth of knowledge to be strapped to one methodology." -Ian Hacking

CORRECT: The ocean is blue because water is a blue substance.

Many people are sure that bodies of water are blue because the water reflects the sky. But wouldn't this only make the shiny surface-reflections look blue? And doesn't water sometimes remain blue on cloudy days? Exactly. There's no mystery here; water looks blue because water *is* blue. Pure water is a blue chemical. It's not just the sky that creates the colors we see.

What color is water?

(Wbeaty youtube 1:08)
But what if you pour yourself a drink; in that case the water is clear, right? Well ...it's not blue as far as your eyes can tell. But what if the water in your cup actually was very very slightly blue. You'd never see it. You'd only notice the blue color if your cup was many feet wide.

In fact, that's exactly how it works: pure water is nearly clear, but it's very very slightly blue. A small amount of water is too thin a layer, so a small amount looks clear rather than blue. But look through thirty feet of water, especially with a white sandy bottom, and you'll see a strong color. Gaze into a hundred feet of deep pure mountain lake water against a white rocky bottom on a sunny day. You'll see exactly what color the water actually has. Yet if you scoop a canteen full of that lake water, it will seem totally clear.

- "Why is water blue?" J. Chem. Edu., 1993,70(8), 612
- Causes of Color (idea.org)
- London South Bank U.: Water absorption spectrum
- Wikipedia: Color of Water

To see some obvious blue, go to the Bahamas and compare the difference between the white sand beaches, the underwater sand in shallow water, and the sand in deeper water:

Or watch a video of a light-colored swimming pool turning blue

CORRECT: The sky is blue because air is blue.

This one isn't purely a textbook error. Still, it involves misconceptions on the part of authors.

Why is the sky colored blue? Usually the books start going on about wavelengths of light, Tyndall effect, and Rayleigh scattering. It's a bit much for young children. First the books try to teach some correct but complicated physics. Then they use it to explain blue sky and sunsets. But what happens when kids don't understand the physics? Is our explanation useless? And do the kids just give up?

It's all wrong: we don't need complicated physics to understand this. The sky is blue for a very simple reason:

The Earth's atmosphere is not a perfectly transparent material. Instead it's blue!
There is no "The Sky." The only material up there is air. Take away the air, and then we'd see what the lunar astronauts see: black daytime sky. No air? No blue. There's no "The Sky" up there; no solid surface. All we're seeing is sunlit air.

A big cloud of air looks blue for much the same reason that a cloud of powder looks white. Powder isn't invisible. Neither is air. Throw some dust upwards on a sunny day and you'll see a visible white cloud. But what happens if you could throw some AIR? You might think that a cloud of air would be invisible. You'd be wrong. Air isn't invisible, instead its molecules scatter light in the same way that any small particles do. Deposit a huge cloud of air onto the surface of the airless moon, and you'd see its bright blue color against the blackness of hard vacuum. Air is a powdery-blue substance. (But then... shouldn't air be a white substance? Yes! And that's where the complicated physics comes in. Once we know that the sky is just a layer of air, and that sunlit air is bright blue, then we can use Rayleigh Scattering to explain why air looks blue rather than white.)

The color of air can be confusing because air seems transparent. Capture a jar full of air, but you see no color. Small amounts of air are almost perfectly transparent. But so are small amounts of water. Go to an opaque muddy river or pond and use a cup to dip out some water. The water in your cup looks fairly clear, no? Yet the deep river is opaque brown. Whenever you try to look through ten cups of water, or a hundred cups, the water seems to turn into opaque brown paint. Yet a single cup of river water ...it almost looks clean! (Aheh, don't drink any.)

Air behaves like water. A mile of air looks clear, but ten miles of air looks misty blue, and a thousand miles of air looks opaque white. The air is acting like the dirty river water where a thin layer looks colorless but a thick layer does not. Air acts like a fogbank where distant objects are invisible, yet you can see your own hand just fine.

"The sky" is blue because air is a powdery blue material; a collection of tiny specks, and when the sun shines upon it, we can see this blue color. Each molecule of air behaves like mote of dust. Stare upwards on a sunny day, and you're looking into a thick cloud of brightly-lit air. (Note: there really is no "sky" up there at all. The sky is an illusory surface. You're not really looking at a blue surface. There is no "sky" which is colored blue, instead you're just seeing the Earth's layer of blue air against the blackness of outer space. )

OK, suppose you could go far out into space away from the Earth, then build yourself a thin hollow glass bubble a thousand miles wide. Viewed from the Earth, your empty glass bubble would be almost invisible. OK, now fill your bubble with air. It won't be invisible any more. It will look like a giant droplet of bright blue paint. It probably even looks whitish in the middle, since very thick layers of air seem as white as milk. What if you let your giant glass bubble crash into the moon? The air inside would pour out over the moon's surface and form a thick temporary layer of atmosphere. The moon wouldn't look white anymore. It would have the same blue borders that Earth has.

Photos of sunlit air, observed against black space:

OK, now here's a question. Smoke is white, milk is white, and powder is white. A big cloud of particles should look like white smoke, not like blue dye. Why is air blue? Shouldn't it look white? And even more important, why are sunsets red? (Does this mean that air is also a red substance?!! I'd have to say yes!) Air is colored reddish for transmitted light, but its color is bluish for reflected light. The color of air is not fixed, instead it's like opal jewelry: the color changes with viewing angle.

Ah, if you start asking why air acts like this, *now* you finally need the advanced physics explanations. Many physics books will explain Rayleigh scattering; explain why an air molecule looks like a bluish dust mote, but looks reddish when lit from behind.

CORRECT: Clouds actually remain aloft because they are warm inside.

Clouds are heavy. Evaporated water (the H2O gas) is not heavy, it actually is less dense than air, so moist air rises. But when the water-gas condenses to form clouds, it contracts by about 1000 times and turns into very dense liquid water. (Imagine that the helium in a balloon condensed into a liquid. Would a tiny liquid-filled balloon still be buoyant? Nope.)

Even a small cloud contains many tons of liquid water. How can clouds remain aloft?

Many sources claim that clouds remain aloft because the water droplets are so small and widely separated that gravity has less effect on them. This is wrong. It doesn't matter if you break up a body of water into tiny droplets; its weight remains the same. You can't fool gravity. If a cloud contains tons of water, it will be pulled down to the Earth's surface with the same force whether the water forms a cloud or whether it forms raindrops. The answer lies elsewhere.

Some sources claim that clouds remain aloft because of updrafts: because the air had been rising, and the rising air blows the cloud droplets upwards. Wrong again: An updraft should be quickly halted as soon as the low-density water vapor turns into a dense liquid. The excess weight will slow the updraft, stop it, then reverse it. To keep clouds aloft, we'd need some sort of weirdly constant updraft, or one where thermal energy is being created, not an updraft that's easily reversed by a falling cloud.

Still other sources claim that clouds stay up there because the droplets are very tiny, so they settle through the air very slowly. This is true, but it still doesn't explain how weighty water can remain aloft. Stop and think a bit... if we have hundreds of tons of water, will its weight disappear simply because it has been divided into tiny droplets? No, instead the heavy droplets drag the surrounding air downwards as they fall. Air which contains water droplets is denser than normal air. Its weight is increased by almost exactly the weight of the suspended water droplets, which works out to around 1/10 percent of the weight of the air in a particular volume.) Dense air falls fast! In other words, the tiny droplets will still race downwards because they form heavy white cloud-stuff, and both the droplets and the air between them will be dragged downwards by gravity. Anyone playing with humidifier fog knows this: dense white pours downwards like a liquid. Yet even some professional meteorologists are saying these things about droplets. They should know better.

So why *DO* clouds stay up there? Why don't they pour downwards to form a ground-hugging fog? The answer is simple: the weight of the cloud's droplets is countered by the buoyancy of heated air between the droplets. Clouds are like hot air balloons!

Whenever liquid water condenses from H2O gas, it releases thermal energy. When moist air turns into droplet-filled air, the droplets are hot, and they warm the air too. The heated air expands and becomes less dense. Is this enough to stop the falling droplets? Yes, it's more than enough, and the warm foggy air flows upwards. Clouds stay up there because they're significantly less dense on average than the surrounding air. In fact, if the water droplets should meld together, then fall out of the cloud as rain, then the remaining hot air is no longer weighed down by tons and tons of water, and it rises even more quickly. This low-density, upward moving warm air is the "engine" which drives the violent updrafts in thunderstorms and hurricanes. Hot air with its water removed no longer floats serenely along as clouds, instead it can form upward jets with hurricane velocity.

Try making this "Touch The Clouds" device and you'll discover that droplet-filled air can be very dense indeed. You can easily pour it from a pitcher and fill some cups. But we also know that hot air is less dense that cool air of the same pressure, so hot must rise through cooler air. Mix the two ideas together: dense air which is full of water droplets becomes less dense when heated, and at a certain higher temperature it should be buoyed upwards by the atmosphere even though it's still full of suspended mass-bearing water droplets. If we could make the humidifier-mist warm enough, it would rise and form indoor ceiling-clouds.

More thinking: helium gas rises in air, but liquid helium does not. Liquid helium is heavy, like liquid water (though not quite as heavy as an equal quantity of water.)

So, what happens when helium gas condenses into liquid? It shrinks greatly, becoming more dense than the surrounding air, then it dribbles downwards like any liquid. It falls downwards if it's a large blob of liquid, and it falls downward even if it takes the form of tiny droplets. If the helium in a balloon was changed into liquid, the balloon would fall. The same is true of water. Water vapor (h2o gas,) like helium, is lighter than air, and it will rise. However, if that vapor should condense into droplets, it greatly contracts in size and greatly increases in density. A cloud of water droplets is heavy, and on average it should fall downwards. Even if the droplets are so tiny that they individually settle slowly, the droplets together have significant weight, so the droplets should drag the air downwards as they go. The dense, droplet-filled air may fall quite quickly, even though the individual droplets remain "stuck in the air" because of forces of viscosity.

Whenever vapor condenses to form droplets, it releases "heat of condensation" which causes the remaining air to expand. The warm air can expand even MORE than the volume left empty by the condensing vapor, causing the average density to fall and causing clouds to rise upwards rather than just float. When clouds first form, they usually pour upwards, not downwards. They're a bit too warm, so they try to rise to a higher level.

  • Wrong: Scientific American "Ask an Expert" Tell them to calculate the heat released by condensation of cloud water, the temperature of resulting air, and the weight of a 1KM cloud compared to 1KM of nearby air which is cooler yet droplet-free.
  • Wrong: New Scientist "Last Word"
  • Wrong: National Geographic Kids
  • Wrong: UK ScienceLine
  • Wrong: Star Tribune: kid's weather questions
  • Wrong: Madsci: ask an expert
  • Wrong: U. Indiana Moment in Science
  • Wrong: U. Corp. Atmos Research
  • Wrong: NASA p.u.m.a.s. (they even mention "lighter than air"... then deny it!
  • Wrong:
  • Wrong: Starbase outreach pgm


  • Steve's Weather FAQ
  • n


    CORRECTED: A single lemon battery cannot light a flashlight bulb

    Some gradeschool science books contain "experiments" which do not work. The prism experiment below is one of them. Another is the "lemon battery" or "potato battery" used to run a flashlight bulb. If you stick some copper and zinc into a single lemon, this "battery" does create a small voltage. Touch your lemon-cell to the wires of a loudspeaker or headphones and you'll hear a clicking sound. Connect it to an old-style panel meter (a voltmeter or milliamp-meter; the kind with the moving needle,) and your lemon can make the meter needle move. Three or four lemon-cells connected in series can run an LCD digital clock or light up a red Light Emitting Diode LED. (If you try the digital clock or LED, remember that polarity is important, and if it doesn't work, try reversing the connections.)

    HOWEVER... the lemon's electrical output is far too feeble to light up a standard flashlight bulb. Same with motors, buzzers, etc. The lemon battery is too weak. The experiment described in the books doesn't work.

    How can I be certain? All those books say one thing, and I'm just one person who says differently. Doesn't the majority rule? No, because science is based on reality staying the same, and Nature ignores what humans vote upon. It doesn't matter how many books say that lemon batteries can light a flashlight bulb. Nature can't be fooled.

    Let's look at a real world example: I stick a fairly wide copper strip and a similar zinc strip into a lemon. (This works much better than copper pennies or zinc nails.) Clean the strips with sandpaper beforehand. First use the strips to tear up the inside of the lemon, then insert the metal strips very close together to give best results. The area of each "battery plate" is around 1 inch square. Measured voltage: 0.91V. Measured short-circuit current: two milliamps (0.002 Amps) immediately decreasing to a constant half a milliamp (0.0005 amps.) What does this mean? Well, a typical flashlight bulb draws an ENTIRE QUARTER OF AN AMPERE when lit. Not a half-milliamp, but 250 milliamps or 0.250 Amps. To light up a normal flashlight bulb, you'd need 500 lemons wired in parallel! 0.2500amps / 0.0005amps = 500 lemons.

    However, there are specialized light bulbs which draw very tiny currents. Maybe the experiments in the books weren't talking about a standard flashlight bulb? (Most of them never say. But I'll give them the benefit of the doubt, although perhaps I shouldn't.) From Radio Shack we can get a #272-1139 incandescent bulb which only draws around fifteen milliamps (0.015 amps) at 0.7 volts when lit very dimly in a darkened room. This is the most sensitive incandescent bulb I've ever encountered. To light this bulb we only need 0.0150A/0.0005A = 30 lemons wired in parallel. THIRTY LEMONS. And the bulb is so dim that you can't see the glow unless the room is dark. But wasn't the lemon's electric current higher at the start? 0.002 amps, not 0.0005 amps? Yes, so with only TEN LEMONS wired in parallel, maybe we could cause the special hyper-sensitive light bulb to blink on for a second or two before going dark.

    This still translates into "the experiment doesn't work." One single lemon cannot light up any sort of incandescent bulb. At best we can use several lemons to light an LED.

    If a science book contains the lemon battery bulb-lightning experiment, it means that the author never performed the experiment to see if it works. LOTS of books and websites say that a single lemon can light a flashlight bulb. Every single one of these is wrong. The mistake is like a kind of infection. If you aren't careful, then your science website can catch a disease!

    Can't we build a larger lemon-juice battery in a jar which will light a small bulb? Yes, but your battery needs to be fairly large; much larger than a couple of metal parts stuck into a lemon. At the very least you'll need a jar for the juice, plus some sheets of copper and zinc several inches wide. If you don't have that special Radio Shack bulb, then you'll need more than one lemon-juice jar hooked in series to make the 1.5 volts needed by a standard flashlight bulb. (I'll try building one of these and report back about how large the copper and zinc plates must be.)

    If you really want to light up a small lightbulb, why not build an ELECTRIC GENERATOR instead?

    How to cheat!

    There is a secret way to make a lemon-cell light up an incandescent bulb. You have to cheat. Buy yourself a "super capacitor" or "memory backup capacitor" via mail-order surplus. They cost a few dollars. You want a value between 0.1 farad and 0.5 farads. Try one of these suppliers: To light a bulb, first build a lemon battery and connect it to the terminals of the supercapacitor. (Me, I use alligator clip-leads bought from Radio Shack.) Wait for a few minutes. Now connect your flashlight bulb to the supercapacitor terminals and it should light brightly for a few seconds. (If not, then remove the bulb and try connecting your lemon cell to the capacitor for 15 minutes to make sure the capacitor gathers enough energy.) The capacitor slowly collects electrical energy from the lemon battery, then it dumps that energy into the flashlight bulb over a very short time. You can even use this trick to let your lemon battery run a low-voltage buzzer or turn a small motor (look for "solar cell motors" from various mail order suppliers or Radio Shack.) As with the bulb, you must charge up the capacitor for many minutes, then use it to run your bulb or motor for a few seconds.

    It's not an ideal experiment, and it's hard to explain how capacitors work. But it's easier than trying to connect thirty lemon-cells in parallel!


    CORRECTED: Ice skates do not function by melting ice via pressure

    It is commonly stated that ice skates have low friction because ice melts when pressure is applied to it. This is not quite correct. A demonstration using an ice cube, a wire, and two weights is often provided to illustrate the phenomena. However, while pressure does affect the melting point of ice, the pressure provided by the skates is not enough to melt ice except when the temperature is a fraction of a degree below 0C. Also, the icecube and wire demonstration is very misleading because it is always performed in a heated room, and the wire doesn't melt ice entirely by pressure, it melts the ice by thermal conduction of warm room temperature along the wire. (Also, narrow gaps in ice always freeze closed because the simultaneous melt/freeze process at water/ice boundary acts to flatten points and fill crevices) Another point: the weight of small objects is too low to create high pressure, yet small objects do experience low friction when on ice. The low friction of ice is probably caused by a layer of liquid water a few hundred molecules thick which always spontaneously develops on the surface of ice. Also, melting from frictional heating can provide liquid water as lubrication. Here's more on this whole debate, and also a bit from BAD CHEMISTRY


    CORRECTED: there are not 92 elements on Earth

    Uranium has the highest atomic number of the elements commonly found in the environment, and some books will tell you that there are 92 elements found on earth: atomic numbers 1 through 92 (hydrogen through uranium). This is wrong. Unfortunately there are two elements below Uranium which are radioactive and have extremely short half lives. These are Technetium and Promethium. These two elements do not occur naturally on Earth, and this reduces the total number of elements found in the environment to 90. However, in the 1970s a natural uranium reactor was found in an ancient streambed in Africa, and the mineral deposits at the site contained traces of a long-lived Plutonium isotope (atomic number 94.) This brings the total number of elements on the Earth back up to 91. (Note: Technetium, though not found naturally on Earth, is present in some stars, detected by spectral analysis.) See THE PHYSICS TEACHER, Vol.27 No.4 p282


    Light from the sun is parallel? Nope.

    Some books state that because the sun is so far away, sunlight arriving at the Earth is almost perfectly parallel. This is incorrect. The book authors reason that, the more distant the object, the more parallel the light, and since the sun is so far away, sunlight is perfectly parallel. They make a mistake. While it is true that light from *each tiny point* on the sun's surface is just about perfectly parallel by the time it reaches our eyes, light from the sun as a whole is not. This is because the sun, though very distant, is very large. A similar situation exists with light from the sky. We wouldn't say that the blue sky emits parallel light. Yet light from the sky comes from many miles away.

    Because the sun is a disk, it creates shadows with fuzzy edges called "penumbras." If sunlight were perfectly parallel, there would be some interesting effects which are usually covered up by these fuzzy edges. First of all, if sunlight was genuinely parallel, then to us the sun would look like a very bright point, like an intensely bright star or a welding arc. Also, shadows on the ground would lack penumbras and be almost perfectly sharp. Without the penumbras, diffraction of light waves would be revealed, and parallel dark and bright lines would appear at the edges of shadows. At nightfall the advancing shadows of distant mountains would be seen to race across the ground. During sunset the brilliant pointlike sun wouldn't gradually sink below the horizon, instead it would wink out. During the day the variations in air density would cause the ground to be covered by moving patterns of light; patterns similar to those seen on the bottom of a swimming pool but in this case made by "waves" in the sky. Solar and lunar eclipses would lack penumbrae. Looking at the sun might burn your retina, since the parallel light would be focused to a tiny point. And if sunlight were perfectly parallel, a large convex lens could concentrate sunlight into an intense pinpoint rather than into a small disk. Also, if a small concave lens were placed near the focus of a large convex lens, the pair lenses could be used to concentrate sunlight and form it into a thin, dangerously powerful parallel beam. Try doing this with the real sun, and all you get is a large, projected image of the sun's disk.

    CORRECTED: with an aircraft wing, the lifting force does not come from the difference in curvature between the top and bottom surfaces.

    First read the entire: wings/lift webpage

    Some books say that the lifting force appears because the wing's upper surface is longer than the lower surface. They state that air dividing at the leading edge of the wing must rejoin at the trailing edge, therefore the upper air stream must move faster, and so the wing is pulled upwards by the Bernoulli Effect. This is not correct: the air divided by the leading edge does NOT rejoin at the trailing edge, and there is no "race" to catch up.

    The same books often contain a misleading diagram showing a flat-bottomed wing with flow lines of the surrounding air. (see below.) This diagram actually shows a zero-lift condition. The lifting force is zero because the air behind the airfoil does not descend. In order to create lift in a three-dimensional situation, a wing must deflect air downwards.

    Both the explanation and the diagram have serious problems. They wrongly imply that inverted flight is impossible. They wrongly imply that an aircraft with a symmetrical wing (a wing with equal pathlengths above and below) will not fly. They also wrongly suggest that an aircraft can violate the conservation of momentum by remaining aloft without reacting against the air, and without causing a downward motion of the air.

    Yet upside-down flight is far from impossible; it is a common aerobatic move. And many wings have equal pathlengths, including even the thin cloth wings of the Wright Brothers' flyer! And anyone standing under a slow, low-flying plane, or below the thin, fast wings of a helicopter will know that there is a very great downward flow of air below the wings. All of this indicates that there is a serious problem with the "curved top, flat bottom" explanation. Below is an alternative.

    Go listen to NPR Science Friday Radio Archive, where physicist D. Anderson debunks the various lifting-force myths.

    Also see the NASA Aerodynamics Education site:

    Here's my attempt at a correct explanation:
    As a plane flies, the leading edges of its wings have little effect on the air, while the trailing edges have a huge effect. The wings' trailing edges always move through the air at an angle. This "effective angle of attack" causes the wing to apply a downward force to the air. In order to create lift, the wing must be tilted. Or rather than being tilted, the wings can be curved or "cambered", which makes the trailing edge of the wing tilt downward at an angle. The trailing edges of the wings cause the departing air to move downwards at an angle. As a result, the wing is pushed upwards and backwards. (These two pushes are called "lift" and "induced drag.")

    The tilted lower surface of the wing causes air to move down, but that's not the only thing. The TOP of the wing also guides the flowing air. This is called "flow attachment" or "Coanda Effect." As the wing moves forward, the air ABOVE the wing moves down, and the wing is forced upwards.

    In other words, as any plane flies, its wings must send a stream of air diagonally downwards, and the wing acts like a 'reaction engine' just like a jet engine or a rocket. Unless a wing is either tilted or cambered, it cannot force the air downwards and cannot generate any "lift."

    It may help to imagine a hovering helicopter: a helicopter can hover because its rotor applies a downward force to the air, and the air applies an upward force to the rotor. As a result, the air flows downwards while the upward force supports the craft. But like any airplane, a helicopter rotor is a moving wing, and it's this small tilted wing which sends the air downwards. Like any wing, helicopter rotors are reaction engines, they push air downwards, and the air pushes them upwards. They are not "sucked upwards," and neither are airplanes.

    You may have seen a plane's downwash of air in movies: a "cropduster" plane sends out a trail of fertilizer mist, and the trail of mist does not float, instead it moves immediately down into the crops, driven downward by the moving air. Air from wings can even be dangerous: if a plane flies too low, the downwash from its wings can knock people over.

    The "Bernoulli effect" is still true. It explains how the top of the wing is able to "pull downwards" on the air flowing over it. And the Bernoulli Effect proves extremely useful in calculations of the lifting force during classes in airplane physics and during experimental work in aerodynamics. But airplanes also obey Newton's laws: accelerate some air downwards, and you'll experience an upwards force.


    Sound travels better through solids? No.

    Many elementary textbooks say that sound travels better through solids and liquids than through air, but they are incorrect. In fact, air, solids, and liquids are nearly transparent to sound waves. Some authors use an experiment to convince us differently: place a solid ruler so it touches both a ticking watch and your ear, and the sound becomes louder. Doesn't this prove that wood is better than air at conducting sound? Not really, because sound has an interesting property not usually mentioned in the books: waves of sound traveling inside a solid will bounce off the air outside the solid. The experiment with the ruler merely proves that a wooden rod can act as a sort of "tube," and it will guide sounds to your head which would otherwise spread in all directions in the air. A hollow pipe can also be used to guide the ticking sounds to your head, thus illustrating that air is a good conductor after all. Sound in a solid has difficulty getting past a crack in the solid, just as sound in the air has difficulty getting past a wall. Solids, liquids, and air are nearly equal as sound conductors.

    It's true that the speed of sound differs in each material, but this does not affect how well they conduct. "Faster" doesn't mean "better." It is true that their transparency is not exactly the same, but this only is important when sound travels a relatively great distance through each material. It's also true that complex combinations of materials conduct sound differently and may act as sound absorbers (examples: water with clouds of bubbles, mixtures of various solids, air filled with rain or snow.) And last: when you strike one object with another, the sound created inside the solid object is louder than the sound created in the surrounding air. So, before we try to prove that solids are better conductors, we had better make sure that we aren't accidentally putting louder sound into the solids in the first place.


    Gravity in space is zero? Wrong.

    Everyone knows that the gravity in outer space is zero. Everyone is wrong. Gravity in space is not zero, it can actually be fairly strong. Suppose you climbed to the top of a ladder that's about 300 miles tall. You would be up in the vacuum of space, but you would not be weightless at all. You'd only weigh about fifteen percent less than you do on the ground. While 300 miles out in space, a 115lb person would weigh about 100lb. Yet a spacecraft can orbit 'weightlessly' at the height of your ladder! While you're up there, you might see the Space Shuttle zip right by you. The people inside it would seem as weightless as always. Yet on your tall ladder, you'd feel nearly normal weight. What's going on?

    The reason that the shuttle astronauts act weightless is that they're inside a container which is falling! If the shuttle were to sit unmoving on top of your ladder (it's a strong ladder,) the shuttle would no longer be falling, and its occupants would feel nearly normal weight. And if you were to leap from your ladder, you would feel just as weightless as an astronaut (at least you'd feel weightless until you hit the ground!)

    So, if the orbiting shuttle is really falling, why doesn't it hit the earth? It's because the shuttle is not only falling down, it is moving very fast sideways as it falls, so it falls in a curve. It moves so fast that the curved path of its fall is the same as the curve of the earth, so the Shuttle falls and falls and never comes down. Gravity strongly affects the astronauts in a spacecraft: the Earth is strongly pulling on them so they fall towards it. But they are moving sideways so fast that they continually miss the Earth. This process is called "orbiting," and the proper word for the seeming lack of gravity is called "Free Fall." You shouldn't say that astronauts are "weightless," because if you do, then anyone and anything that is falling would also be "weightless." When you jump out of an airplane, do you become weightless? And if you drop a book, does gravity stop affecting it; should you say it becomes weightless? If so, then why does it fall? If "weight" is the force which pulls objects towards the Earth, then this force is still there even when objects fall.

    So, to experience genuine free fall just like the astronauts, simply jump into the air! Better yet, jump off a diving board at the pool, or bounce on a trampoline, or go skydiving. Bungee-jumpers know what the astronauts experience.

    Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. --Fred Hoyle

    CORRECTED: For every action, there is not an equal and opposite reaction.

    Newton originally published his laws of motion in Latin, and in the English translation, the word "action" was used in a different way than it's usually used today. It was not used to suggest motion. Instead it was used to mean "an acting upon." It was used in much the same way that the word "force" is used today. What Newton's third law of motion means is this:

    For every "acting upon", there must be an equal "acting upon" in the opposite direction.
    Or in modern terms...
    For every FORCE applied, there must be an equal FORCE in the opposite direction.
    So while it's true that a skateboard does fly backwards when the rider steps off it, these motions of "action" and "reaction" are not what Newton was investigating. Newton was actually referring to the fact that when you push on something, it pushes back upon you equally, even if it does not move. When a bowling ball pushes down on the Earth, the Earth pushes up on the bowling ball by the same amount. That is a good illustration of Newton's third Law. Newton's Third Law can be rewritten to say:
    For every force there is an equal and opposite force.

    Or "you cannot touch without being touched."

    Or even simpler: Forces always exist in pairs.


    CORRECTED: Ben Franklin's kite was never struck by lightning

    Many people believe that Ben Franklin's kite was hit by a lightning bolt, and this is how he proved that lightning is electrical. A number of books and even some encyclopedias say the same thing. They are wrong. When lightning strikes a kite, the electric current in the string is so high that just the spreading electric currents in the ground can kill anyone standing nearby, to say nothing of the person holding the string! What Franklin actually did was to show that a kite would collect a tiny bit of electrical charge-imbalance out of the sky during a thunderstorm.

    Air is not a perfect insulator. The charges in a thunderstorm are constantly leaking downwards through the air and into the ground. Electric leakage through the air caused Franklin's kite and string to become charged, and the hairs on the twine stood outwards. The twine was then used to charge a metal key, and tiny sparks could then be drawn from the key. Those tiny sparks were the only "lightning" in his experiment. (He used a metal object because sparks cannot be directly drawn from the twine; it's conductive, but not conductive enough to make sparks.)

    His experiment told Franklin that some stormclouds carry strong electrical charges, and it implied that lightning was just a large electric spark.

    The common belief that Franklin easily survived a lightning strike is not just wrong, it is dangerous: it may convince kids that it's OK to duplicate the kite experiment as long as they "protect" themselves by holding a silk ribbon and employing a metal key. Make no mistake, Franklin's experiment was extremely dangerous. Lightning goes through miles of insulating air, and will not be stopped by a piece of ribbon. If lightning had actually hit his kite, he would have been gravely injured, and most probably would have died instantly. See LIGHTNING SURVIVOR RESOURCES


    The main lens of your eye is inside the eye? Not quite.

    Some textbooks assume that the small lens found deep within the eyeball is the eye's main lens, and the cornea of the eye is simply a protective window. The textbook diagrams even depict light rays passing into the eye and only bending as they pass through this internal lens. But in the human eye, the small lens found within the eyeball is not the main imaging lens. The cornea is actually the main lens; it is the strongly curved transparent front surface of the eye. Most of the bending of the light occurs at the place where the light enters the surface of the cornea. When you look at your eye in the mirror, you are looking directly at the eye's main lens. When you want to change the focusing power of your eye, you apply "contact lenses" to the cornea surface, or you undergo surgery which re-sculpts the cornea's curvature. The smaller lens inside the eye acts only to alter the focus of the eye as a whole. Muscles change its shape in order to correct the focus for near and far viewing. Without this small internal lens, human vision would be blurry, and vision would be unable to accommodate for near and far views. But without the cornea lens, [the human eye would be blind.] IMPROVED VERSION: without the cornea lens, human vision would rely upon the pinhole-camera effect of the eye's pupil, and vision would be incredibly blurry. Open your eyes underwater in dimly-lit conditions to see what vision would be like without a cornea.


    CORRECTED: when one prism splits light into colors, a second identical prism cannot recombine them.

    A single prism can split a sunbeam into a rainbow. Many children's science books show how a second similar prism can be used to recombine the colors. This is incorrect, two prisms do not work as shown. Prisms of two different sizes can split and then focus the colors into momentary recombination at a particular distance. With three prisms in a special arrangement, the splitting and complete recombining of colors can be accomplished. But books which depict one prism splitting the colors and a second identical prism recombining the colors into a single white beam are in error, and are no doubt the source of endless frustration for those of us who try to duplicate the effect with real prisms.

    The "rainbows" can also be recombined by placing a screen at just the right place, and by bouncing the colors off many small mirrors so the colored beams converge upon a screen. Recombination can also be done with a convex lens or a concave mirror and a screen. I hope that very few students will attempt to perform the color recombination experiment depicted in their books, for disappointment awaits. (MORE)


    Clouds, fog, and shower-room mist are water vapor? No.

    All three things are made of small droplets of liquid water hanging in the air. When water evaporates, it turns into a transparent gas called "water vapor." When it condenses again, it can take the form of rain, snow, rivers, and oceans, but it also can take the form of clouds, mist, fog, etc. Fog can make surfaces wet, but not because of condensation. Instead, the fog droplets collide with the solid surface. Fog is liquid water, not a vapor. Fly an ultralight aircraft slowly through a large dense cloud, and you'll become damp. To look for water vapor, look at the bubbles in rapidly boiling water. Look at the small empty space at the spout of a boiling teakettle. Look at the far end of the teakettle's plume of mist, where the mist seems to vanish into the air. Look at the empty air above a wet surface. In these situations you see nothing, and that's where the vapor is. Water vapor seems invisible because it is transparent. Clouds and fog are not transparent. They are composed of liquid droplets.


    CORRECTED: raindrops don't have points!!

    Nearly every drawing of raindrops depicts them as having a sharp upper point. This is wrong. Surface tension of water acts like a stretched "bag" around the water, and unless some other force is acting, it pulls the water into a spherical shape. Our eyes do see tiny droplets as a blur, but a flash photograph reveals that small raindrops are nearly spherical. The larger ones are distorted by the pressure of moving air, but this doesn't make points, it makes them somewhat flattened. Think of it this way: underwater bubbles are not pointed as they rise, just as falling water drops are not pointed as they fall. And while it's true that the symbol for water is a droplet with a point, real water droplets look nothing like the symbol. And when water drips from a faucet, it never actually has a point. Instead it has a narrow neck, and after the neck has snapped, it is yanked back into the falling ball of water. See Dr. Fraser's BAD SCIENCE for lots more about this.


    Air is weightless? No.

    We are not conscious of air's weight because we are immersed within it. In the same way, even a large bag of water seems weightless when it is immersed in a water tank. The bag of water in the tank is supported by buoyancy. In a similar way, buoyancy from the atmosphere makes a bag of air seem weightless when it's surrounded by air. One way to discover the real weight of air would be to take a bag of air into a vacuum chamber. Another way is to weigh a pressurized and an unpressurized football. A cubic meter of air at sea-level pressure and 0C temperature has a mass of 1.2KG. The non-metric rule of thumb says that the air that would fill a bathtub weighs about one pound. Here's a simple way to detect the mass of air even though the air seems weightless: open an umbrella, wiggle it slightly forwards and back, then close it and wiggle it again. When you wiggle it when open, you can feel its increased mass because of the air the umbrella must carry with it. (Ah, but then we must explain the difference between weight and mass!)


    CORRECTED: Filled and empty balloons do not demonstrate the weight of air.

    Many books contain an incorrect experiment which purports to directly demonstrate that air has weight. A crude beam-balance is constructed using a meter stick. Deflated rubber balloons are attached to the ends, and the balance is adjusted. One balloon is then inflated, and that end of the balance-beam is supposed to sag downwards. The demonstrator then explains that a large amount of air weighs more than a small amount of air.

    Unfortunately this experiment isn't very honest. When immersed in atmosphere, buoyancy causes full and empty balloons to weigh the same. One balloon shouldn't pull down the stick. But then why does the above experiment work? Usually it doesn't! In fact, the experiment will fail unless you know the trick: you must inflate the balloon near to bursting. The experiment secretly relies on the fact that the air within a high-pressure balloon is denser than air within a low pressure balloon. Of course the demonstrator never mentions this to the students, and the books which contain this demonstration don't mention density effects either. Obviously the density effects do not directly demonstration anything about the weight of air, so it's dishonest to tell students that this demonstration can directly weigh some air.

    To illustrate the problem, try this instead: attach two opened paper bags to the balance, adjust it, then crush one bag so it contains little air. The balance will not change. What does this teach your class; that air is... weightless? Yet the air does have significant weight. We just can't detect this weight directly by using paper bags. Or using balloons.

    Here's a way to make the experiment more honest. Perform the balance-beam experiment again, but use two full balloons. Blow one balloon really full so the rubber feels hard and the balloon is about to pop. Blow up the second balloon so it is almost full, but still a bit stretchy. Try to keep the balloons almost the same size. Now the balance will show that, even though the balloons look nearly the same, the "hard" balloon is significantly heavier. Does this teach misleading things to your class? Not really, instead it exposes the dishonesty of the original demonstration. In truth, balloons filled with air will not weigh more than empty balloons as long as they remain immersed in the atmosphere. However, compressed air does weigh more than uncompressed air. Perhaps this modified demonstration would be appropriate in more advanced classes. But this website is about K-6 grade science.

    Here's another way to think about it. Can we demonstrate the weight of water to a fish? What if we lived underwater, how could we use the balance-beam to measure the weight of water directly? The answer is that we cannot. If a water-filled balloon and a collapsed empty balloon were compared underwater, the experiment would show that they weigh the same, which seems to prove that water is weightless. When underwater, a bag full of water weighs just the same as a flattened bag which contains nothing. The situation with air is similar: if we live our lives immersed within a sea of air, we cannot use a balance to easily detect the actual weight of the air. (In fact, a bathtub full of air weighs about a half kilogram, but we cannot easily demonstrate this weight while living in an atmosphere.)

    It's hard to teach the weight of water to the fishes, and hard to teach the weight of air to human grade-schoolers. These misleading textbook experiments could only work correctly if performed in a vacuum environment (say, on the moon's surface.) We humans are like fish underwater: we're not aware that our ocean of air has any weight. To demonstrate the weight, we need to get out into a vacuum environment.

    Or, to better demonstrate the weight of air directly, hook a heavy bottle to a vacuum pump, pump all the air out, seal it, then weigh the bottle. Break the seal and let the air in, then weigh it again. The difference in weight is the weight of the air contained in the bottle. Another: use a balance to compare the weight of two vacuum-containing bottles, then open one of them so it becomes filled with air. The bottles will then weigh differently, and the difference is the true weight of the air in one bottle. Or another: build a balance using upside-down paper bags, then place a candle below one of them, then remove the candle again. That bag rises, indicating that a volume of warm air weighs slightly less than a volume of cool air. (Don't set the bag on fire!!) But note that this candle experiment is a bit like the compressed balloon version, and it says nothing simple and direct about the actual weight of a volume of unheated air.


    CORRECTED: in the everyday world, gases do not expand to fill their containers.

    What is the difference between a liquid and a gas? Both are "fluids", both can flow. Gases are usually less dense than liquids, although gases under fiercely high pressure can approach the density of liquids, so that's not a good criterion. The main difference is that gases are a different phase of matter: a gas can be made to condense into a liquid form, and a liquid can be made to evaporate into gas. Another major characteristic: because there are bonds between its particles, when a liquid is placed into a vacuum environment, it will not immediately and continuously expand, while a gas in a vacuum chamber will expand at high velocity until it hits the container walls.

    This is very different from the oft-quoted rule that "gases always expand to fill their containers." This rule only works correctly if the container is totally empty: the container must "contain" a good vacuum beforehand. However, we all live in a gas-filled environment. All our containers are pre-filled with air. In our environment, any new quantity of gas will not expand, it will just sit there. It may slowly diffuse outwards, but that's very different than the "expansion" being discussed here. If you squirt some carbon dioxide out of a CO2 fire extinguisher, it will not instantly expand to fill the room. Instead it will pour downwards like an invisible fluid and form a pool on the floor. It behaves similarly to dense sugar-water which was injected into a tank of water: it pours downwards, and only after a very long time it will mix with the rest of the water. "Mixing" is very different from "expanding to fill!" The rule about gases does not involve mixing; instead it involves compressibility and instant expansion into a vacuum.

    In an air-filled room, dense gases act much like liquids; they can be poured into a cup or bowl, poured out onto a tabletop, and then they run off the edge onto the floor where they form an invisible mess. :) Less dense gases will stay where they are put, like smoke or like food coloring which has just been injected into a fishtank. Gas of even lesser density rises and forms a pool on the ceiling. Only in the world of the physicist, where "empty container" always implies a vacuum, does the rule about gasses work properly.


    CORRECTED: Shadows do vanish on cloudy days, but not because the sun isn't bright enough.

    Shadows appear when an object blocks a light source. The shape of the shadow is created by the shape of the opaque object and by the shape of the light source. On a cloudy day the whole sky acts as a light source, and a person's shadow spreads out and becomes a dim fuzzy patch which surrounds the person on the ground on all sides. The shadow is so spread-out that it seems absent entirely. When the sun is visible, the same shadow is concentrated in one specific place and becomes easy to see. But even the shadows made by sunlight will have fuzzy borders, since the sun is a small disk rather than a tiny dot. On cloudy days, the fuzzy borders of your body's shadow become much much larger than the shadow itself, so that the shadow seems to vanish.



    Some books point to surface roughness as the explanation of sliding friction. Surface roughness merely makes the moving surfaces bounce up and down as they move, and any energy lost in pushing the surfaces apart is regained when they fall together again. Friction is mostly caused by chemical bonding between the moving surfaces; it is caused by stickiness. Even scientists once believed this misconception, and they explained friction as being caused by "interlocking asperites", the "asperites" being microscopic bumps on surfaces. But the modern sciences of surfaces, of abrasion, and of lubrication explain sliding friction in terms of chemical bonding and "stick & slip" processes. The subject is still full of unknowns, and new discoveries await those who make surface science their profession

    When thinking about friction, don't think about grains of sand on sandpaper. Instead think about sticky adhesive tape being dragged along a surface.



    Infrared light is invisible light.

    When any type of light is absorbed by an object, that object will be heated. The infrared light from an electric heater feels hot for two reasons: because surfaces seem black to IR light, and because the IR is extremely bright light. Just because human eyes cannot see the light which causes the heating does not mean it's made of some mysterious entity called "heat radiation." When bright light shines on an absorptive surface, that surface heats up.

    And this is no benign misconception. Those who fall under its sway may also come to believe that *visible* light cannot heat surfaces (after all, visible light is not "heat radiation?") Misguided science students may wrongly believe that warm objects emit no microwaves (since only IR light is "heat radiation"), even though hot objects actually do emit microwaves. Or students may believe that the glow of red hot objects is somehow different from the infrared glow of cooler objects. Or they may believe that IR light is a form of "heat," and is therefore fundamentally different from any other type of electromagnetic radiation.

    In his book " Clouds in a Glass of Beer," Physicist C. Bohren points out that this "heat" misconception may have been started long ago, when early physicists believed in the existence of three separate types of radiation: heat radiation, light, and actinic radiation. Eventually they discovered that all three were actually the same stuff: light. "Heat radiation" and "actinic radiation" are simply invisible light of various frequencies. Today we say "UV light" rather than "actinic radiation." Yet the obsolete term "heat radiation" still lingers. Since human beings can only see certain frequencies of light, it's easy to see how this sort of confusion got started. Invisible light seems bizarre and mysterious when compared to visible light. But "invisibility" is caused by the human eye, and is not a property carried by the light. If humans could see all the light in the infrared spectrum, we would say things like this: "of course the electric heater makes things hot at a distance, it is intensely bright, and bright light can heat up any surface which absorbs it."

    PS, if you're interested in physical science misconceptions, Bohren's Book is an excellent resource. He's like me, and complains about several specific misconceptions which keep his students from understanding science.



    Actually there's a very large number of distinct colors in any rainbow. And neither are there sharp divisions between the bands of color, yet numerous textbooks depict them. In reality, between yellow and green we find yellow-green, and between green and yellowgreen is greenish yellowgreen, and on and on. How many colors are in a rainbow? Thirty? Sixty? It's not easy to say, for it depends on the particular eye, and the particular rainbow. What of the teachers and students who look in vain for the yellow-green in their textbook's depiction of rainbows? They've crashed into a long-running textbook misconception: the strange idea that rainbows have exactly seven distinct bands of color and no more, and with nothing in between those uniform bands of 'official' color.



    Many textbooks have an erroneous diagram of the earth which shows a bar magnet within it, and the ends of this bar magnet extend to just beneath the earth's surface. These diagrams depict the magnet's field lines as radiating from spots on the earth's surface. This is very misleading. The earth's magnetic poles actually behave as if they're deep within the earth, down inside the core. The Earth's magnetic field does not come from a giant bar magnet, but if we imagine that it does, then the imaginary "bar magnet" inside the earth is short, stubby, disk-shaped, and part of the iron core deep inside the planet.

    The typical textbook diagram is incorrect, and there are no intense magnetic fields at the land surface near the earth's "north pole" and "south pole." If you stand at the Earth's south magnetic pole, metals aren't attracted to the ground more strongly than anywhere else. The Geomagnetic "poles" on the earth's surface are not places where the field is strong. They are simply the points on the landscape where the field lines are perfectly vertical.

    Proper diagrams should instead show the field lines to be radiating from poles inside the earth's core. They should show the field lines around the northern and southern areas of the earth's surface as being approximately vertical and parallel, not "radial" like a spiderweb and not concentrated into special points on the surface.

    Another error associated with the above: some books claim that the earth's field at the magnetic poles is much stronger than elsewhere. This is untrue. The field strength at the north magnetic pole above Canada is about the same as the field strength in Virginia! And the strongest field in the Earth's northern hemisphere does not appear at the north magnetic pole at all, the North Pole actually has a weaker field than elsewhere. The strongest fields in the northern hemisphere are not in one but in two places: west of Hudson Bay in Canada, and in Siberia.




    It's incorrect to say that "in laser light the waves are all in phase." When two light waves traveling in the same direction combine, they inextricably add together, they do not travel as two independent "in-phase" waves. The photons in laser light are in phase, but the WAVES are not. Instead, ideal laser light acts like a single, perfect wave.

    When the light wave within a laser causes atoms to emit smaller, in phase light waves, the result is not "in phase" light. Instead the result is a single, more intense, amplified wave of light. In-phase emission leads to amplification, not to multiple in-phase waves. If the atoms' emissions weren't in phase, the result would not be light that's out of phase. Instead the atoms would absorb light rather than amplifying it.

    Each atom in a laser contributes a tiny bit of light, but their light vanishes into the main traveling wave. The light from each atom strengthens the main beam, but loses its individuality in the process. 99 plus 1 equals 100, but if someone gives us 100, we cannot know if it is made from 99 plus 1, or 98 plus 2, or 50 plus 50, etc.

    Yes, it's true that all the photons associated with a single wave of light are in phase. This might be one reason that people say that laser light is "in phase" light. However, in-phase photons are nothing unique, and they don't really explain coherence. Any EM sphere-wave or plane-wave is made of in-phase photons. For example, all the photons radiated from a radio broadcast antenna are also in phase, but we don't say that these are special "in phase" radio waves, instead we just say that they are waves with a spherical wavefront. Even if all the photons in laser light are in phase, it is still incorrect to say "all the WAVES are in phase." Photons are not waves. They are quanta, they are particles, and they do not behave as small, individual "waves." Yes, all the photons are in phase, but only because they are part of a single plane-waves.

    The light from a laser is basically a single, very powerful light wave. Single waves are always in phase with themselves, but it's misleading to imply that a single plane-wave or sphere-wave is something called an "in phase" wave. Laser light could more accurately be called "pointsource" light. Sphere waves or plane waves behave as if they were emitted from a single tiny point. The physics term for this is "spatially coherent" light. Light from light bulbs, flames, the sun, etc. are the opposite, and are called "extended-source" light. Extended-source light comes from a wide source, not from a point-source, and the waves coming from different parts of the source will cross each other. Starlight and the light from arc welders is "point-source" light and is quite similar to laser light. Light from arc-welders and from distant stars has a higher spatial coherence than light from most everyday light sources. (Note: the sun is a star, correctly implying that light becomes more and more spatially coherent as it moves far from its source. This is a clue as to the real reason that lasers give spatially coherent light! (See below)



    Light from most lasers is not parallel light. However, if laser light is passed through the correct lenses, it can be formed into a tight, parallel beam. The same is not true for light from an ordinary light bulb. If light from a light bulb were passed through the same lenses, it would form a spreading beam, and an image of the lightbulb would be projected into the distance. Laser light can form beams because a laser is a pointsource, and when you project the image of a pointsource into the distance, you form a narrow parallel beam! However, it is simply wrong to state that laser light is inherently parallel light. Laser light can be formed into parallel light, while the light from ordinary sources cannot.

    Most types of lasers actually emit spreading, non-parallel light. Lasers in CD players and in "laser pointers" are semiconductor diode lasers. They create cone-shaped light beams, and if a parallel beam is desired, they require a focusing lens. The same is true for the lasers in inexpensive "laser pointers." Take apart an old laser-pointer, and you'll find the plastic lens in front of the diode laser inside.

    Classroom "HeNe" lasers also create spreading light. The laser tube within a typical classroom laser contains at least one curved mirror (called a "confocal" arrangement,) and it creates light in the form of a spreading cone. It's a little-known fact that manufacturers of classroom lasers traditionally place a convex lens on the end of their laser tubes in order to shape the spreading light into a parallel beam. While it's true that a narrow beam is convenient, I suspect that part of their reason is to force the laser to fit our stereotype that all lasers produce thin, narrow light beams. The manufacturers could save money by selling "real" lensless laser tubes having spreading beams. But customers would complain, wouldn't they? We have been brought up to believe that laser light is parallel light.



    In-phase emission causes the amplification of light, it doesn't cause coherent light. Because the atoms emit light in phase with incoming light, they will amplify the light, but they amplify incoherent light too, and they don't make it coherent. The coherence of laser light has another source... Laser light has two main characteristics: it is "monochromatic" or very pure in frequency (this also is called "temporally coherent.") Laser light also has a point-source character of sphere waves and plane waves (also called "spatially coherent.")

    Even fairly advanced textbooks fail to give the real reason why laser light is spatially coherent. They usually point out that the laser's atoms all emit their light in phase, and pretend that this leads to spatial coherence. Wrong. It is true that the fluorescing atoms in a laser all emit light that's in-phase with the waves already traveling between the mirrors. But the in-phase emission only creates amplification of the traveling waves, it does not create spatially coherent light. For example, if you were to feed incoherent light into a HeNe laser tube, the atoms would emit in-phase waves, and the laser would amplify the light. But the brighter light would still be incoherent! Lasers certainly can amplify the coherent wave which is trapped between their mirrors. But how did the light within the laser get to be coherent in the first place?

    Lasers create coherent light because of their mirrors.

    The mirrors in a laser form a resonant cavity which preserves coherent light while rejecting incoherent light. How does it work? Imagine a simplified laser having flat, parallel mirrors. As light bounces between the mirrors, the light "thinks" that it's traveling down an infinitely long virtual tunnel. (Have you ever held up two mirrors facing each other? Then you've seen this infinite tunnel.) When a laser is first turned on, it fluoresces; it emits light which is not coherent. Different random light waves start out from different parts of the laser. After a few thousand mirror bounces, all the waves have added and subtracted to form just one single EM wave. In the case of flat-mirror lasers, this wave is a nearly perfect plane wave. A single plane wave is coherent (to be incoherent, you must have at least two different waves.)

    This can be a bit confusing. After all, the individual atoms each emit a wave. Don't all these waves add up to messy incoherent light? No. The in-phase emission preserves the existing coherence as it amplifies. It's true that each atom emits light waves in all directions. However, these sideways waves from all the atoms will cancel each other out, and only the waves that travel in the same direction as the incoming light will be preserved. It's as if the atoms "know" which direction to send out their beam. But in reality, the atoms don't need to know the beam direction. Instead, they just emit a light wave which is in phase with the incoming light, and for this reason the wave from the atom will cancel out everywhere except in a line with the incoming light. If the light in a laser were already coherent, then the atoms will amplify it but won't make it more coherent. The coherence comes from the great distance that the light has traveled as it bounced between the mirrors.

    A similar thing happens with starlight: starlight is coherent! Starlight travels far from its original source and all the waves from different parts of the star will add up to form a wave with a single planar wavefront. Light from distant stars is spatially coherent, even though sunlight is not, yet the sun is a star too. The farther the light travels from its source, the more it approaches the shape of a perfect plane wave. And a perfect plane wave is perfectly coherent. Laser light is spatially coherent because, among other things, the bouncing light has traveled millions of miles between mirrors, and all the various competing waves have melded together to form a single pure plane-wave or sphere-wave.

    P.S. The pure color (monochrome) laser light is also created by the mirrors. Huh? Yes, but the reason for this is not totally straightforward (and it's quite a bit beyond the K-6 level of these webpages!)

    The two mirrors of a laser can trap a standing wave of light. The space between the mirrors is like the string of a guitar: there can be a fundamental wave, or overtone waves, or complicated waves which are a mixture of these. But waves of non-overtone frequencies cannot exist between the mirrors. Since the distance between the crests of a light wave is very small, lots of different overtones can fit between the mirrors, and each overtone is a slightly-different pure color of light. Light from a neon sign is reddish, but it doesn't have the extreme purity of laser light. Now for the weird part: when a Helium-Neon laser first operates, many different overtones of red light are amplified and the beam contains many slightly-different colors of red at the same time. It's not yet monochromatic. As time goes on, some of these colors are amplified a bit more than others, and this uses up the available energy coming from the laser power supply. In other words, the different waves start competing for limited resources! Just one wave "wins" in the end, and all of the other overtones drop out of the running. The laser light is not just red light. Instead it is a single pure overtone-wave, a pure frequency where the string of waves just perfectly fits in the space between the two mirrors. Change the spacing of the laser's mirrors (for example by heating a glass HeNe laser tube,) and you change the frequency of the light.



    There are numerous others. Nickel and Cobalt metals are very magnetic. (U.S. "nickel" coins contain copper which spoils the effect, so try Canadian nickels made before 1985.) Most other materials are "diamagnetic," and are repelled visibly by very strong magnets, although some materials are "paramagnetic" and are attracted. Supercold liquid oxygen is attracted by magnets. Some but not all types of stainless steel are nonmagnetic. There are even some metals which are individually nonmagnetic, but which become strongly magnetic when mixed together, chromium and platinum for example, and compounds of manganese and bismuth.


    They are heated as they plow into the atmosphere and compress the air ahead of them. Ever pump up a bicycle tire and discover that the pump and the tire have become hot? The same effect causes spacecraft and supersonic aircraft to heat up as they compress the air at their leading edges. The heat doesn't come from *rubbing* upon the air, it comes from *squeezing* the air. This applies mostly to blunt objects such as Apollo reentry vehicles. It does not apply as much to the Space Shuttle: with wings oriented mostly edge-on to the moving air, the surfaces of the Shuttle are heated by friction. But when the Shuttle first reenters the atmosphere, the bottom of the craft faces forwards, and in that case the Shuttle is heated by air compression, not by friction.



    They are slowed because it takes energy to stir the air. While direct friction between the air and the car's surface does play a part, the work done in stirring the air far exceeds the work done in direct frictional heating. If vehicles did not send air swirls and vortices spinning off as they moved, they would barely be slowed by the air at all. Eventually the swirling air is slowed by friction and ends up warmer, but this occurs long after the vehicle has passed.



    Opposite poles attract. If we hold two bar magnets near each other, the "N" pole of one magnet is attracted by the "S" pole of another. If we suspend a bar magnet by a thread, the "N" pole of that magnet will point... toward the Earth's north!

    Something is wrong here. Shouldn't the "N" pole of a magnet point towards the "S" of the Earth? Alike poles should repel, not attract. Either the "N" and "S" printed on all bar magnets is reversed, or the "N" and "S" on the Earth is backwards. Which is it?

    This problem has a simple solution. Physicists define "N-type" magnetic poles as being the north-pointing ends of compasses and magnets. This definition is built into all of modern science and engineering and is part of Maxwell's equations. Wind an electromagnet coil, see which end points towards the Earth's North Pole, and that end is the "N pole" of the electromagnet. And this means that the magnetic pole found deep inside the northern hemisphere of the Earth is a south-type magnetic pole. The Earth's northern magnetic pole is an S! It has to be this way, otherwise it would not attract the N-pole of a compass.

    This is a long-standing but arbitrary physical standard, much the same as defining electrons as being negative. Like it or not, we are stuck with negative electrons, with seconds which last about 1/100,000 of a day, with backwards Earth poles, with centimeters which are about as wide as a small finger, etc.

    Interesting email msgs on magnetic polarity
    See Dexter Magnetics for more on this.
    Also try this Google search



    Salt is not made of NaCl molecules. Salt is made of a three-dimensional checkerboard of oppositely charged atoms of sodium and chlorine. A salt crystal is like a single gigantic molecule of ClNaClNaClNaClNaClNaClNa. When salt dissolves, it turns into independent atoms. Salt water is not full of "sodium chloride." Instead it is full of sodium and chlorine! The atoms are not poisonous and reactive like sodium metal and chlorine gas because they are electrically charged atoms called "ions." The sodium atoms are missing their outer electron. Because of this, the remaining electrons behave as a filled electron shell, so they cannot easily react and form chemical bonds with other atoms except by electrical attraction. The chlorine has one extra electron and its outer electron shell is complete, so like sodium it too cannot bond with other atoms. These oppositely charged atoms can attract each other and form a salt crystal, but when that crystal dissolves in water, the electrified atoms are pulled away from each other as the water molecules surround them, and they float through the water separately.



    They only travel at the "speed of light" (186,000 miles per second) while moving through a perfect vacuum. Light waves travel a bit slower in the air, and they travel lots slower when moving through glass. Why does light bend when it enters glass at an angle? Because the waves SLOW DOWN. Why can a prism split white light into a spectrum? Because within the glass the different wavelengths of light waves have different speeds And while the numerical value for the speed of light in a vacuum, "c," is very important in all facets of physics, as far as light waves are concerned there is no single unique speed called "The Speed Of Light." [note for advanced students: ok ok, I'll add this: light *waves* within a transparent medium are slow, even though the wave's photons are thought to jump from atom to atom always at a speed of c. But such ideas are not very honest, since whenever we only pay attention to the vacuum between particles in a solid, we stop treating the solid as a "uniform transparent medium." ]


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