Question Board -- Questions about Light

These are questions and reports sent in by participants in the Virtual Workshop on Light, and comments on them. If you have a question or an answer or a comment, send it in!
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Questions about temperature and heat
Questions about electricity and magnetism
Questions about other subjects

The questions (there may be more on the pages) (Click on a line to jump to the entry)
Questions about light:
If a flashlight has low batteries, how far will its light beam travel?
Why is glass invisible when immersed in oil?
Can the edge of a shadow move faster than the speed of light?
Why don't extended sources of light make sharp shadows?
Amplifying light with mirrors
Shadows on the moon
Why is moonlight cool?
Taking photographs through a window
Why do objects appear to get smaller as you move away from them?
Shapes of Long Shadows
Doesn't the divergence of the sun's ras prove that the sun is a lot closer than 93 million miles?"
Size of an airplane's shadow
I can't darken my classroom
Using flashlights as light sources
About large screen TVs
Why is the diffraction grating spectrum symmetrical?
Why isn't the sky blue on moonlit nights?"
Why does light cause colors to fade?
What is a light beam made of?
How can I display beams of light in a dim room?
How can I make a curved mirror?
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My Question: How far will a light beam travel? If there was an extremely strong light beam that could be seen for miles, where would the light stop? If a flashlight has low batteries, how far will its light beam travel?
Joe's explanation Forever and ever and ever! Each little bit of light travels in a straight line, to the end of the universe. That's why we can see the stars. But I think you meant something else: from how far away can you still see a dim flashlight? Now there are three issues:
*No light beam is perfectly collimated -- that is, to some extent it contains light that is traveling in different directions. Then as you get farther and farther away, the beam is spread out over a larger area, and is correspondingly dimmer. The stars are as bright as the sun, when you are close to them, but very distant stars are hard to see. A laser makes a pretty good beam, but the best laser beam we know how to make is a mile wide by the time it arrives at the moon. A man on the moon would not be able to see it -- it would be too dim.
*If we try to do the experiment on earth, we have to deal with the atmosphere, which is not completely transparent. A little bit of haze (small droplets of water or something), dust, bugs, or even temperature variations in the air can affect the light beam. If the light is being scattered into other directions, it is ruining the collimation. If the light is being absorbed, it's just gone.
*Finally, when we say we can't see the light, what we usually mean is that we see something else. We can't see through fog because it is bright -- it is scattering light into our eyes which blots out the light that is trying to go directly through the fog. The result is that on a foggy day, a distant building can barely be seen -- there is only a slightly different shade of gray against the sky behind it. The gray is the light the fog is sending; the slight extra brightness of the sky is the light that has made it through the fog to show us the scene beyond. This is also why most people can't see the stars at night: they are hidden behind the glow in the sky caused by the lights of the city they live in.

Putting all this together: the brightness of the light source and how well it is collimated are important, but the real answer to your question lies in factors outside the light source. The light beam doesn't stop -- it just gets harder and harder to see, and eventually disappears into the background.
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My Question: Why is glass invisible when immersed in oil? When a glass rod is inserted into a bottle of cooking oil, it becomes hard to see. What is going on?
Sally's explanation We see transparent objects in two ways: light is reflected from the surfaces, and light changes direction as it goes through them. The first gives a glint from some surface, if it is curved; the second gives a distortion of the view that you would otherwise see, and also causes a shadow, because light doesn't end up where it should. You can see all three of these effects in this picture, though the background is so uniform that this effect is not very pronounced.

Both the change in direction and the reflection arise because the optical properties of the glass and the air are different. However, the optical properties of oil is much more similar to that of glass, so that these effects are less important. So a glass object in cooking oil is hard to see. And if you choose pyrex glass (and not the more common "soft" glass), and Wesson cooking oil (and not just anything off of the shelf) the match becomes extremely good. There is no reflection at the surface, and the change in direction is very slight -- the object is almost invisible.

The part of the glass rod that is still in the air is just as visible as ever, since there the light is moving from air to glass and back again. It seems to abruptly end as it enters the oil. The container just looks like it is full of oil -- because it is full of stuff that has the same optical properties (specifically, the same index of refraction).
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My Question:"Can the edge of a shadow move faster than the speed of light?
Joe's explanation
Yes indeed. The real rule is that no information can be transmitted from one place to another faster than the speed of light. Objects can carry information, and a flash of light carries information, so they have to obey this rule; but the edge of a shadow or the edge of a light beam doesn't carry any information. Consider this situation:

You point your flashlight at the red star, and then switch it to the purple star a second later. Travelling at the speed of light, it takes a year for the signals to get to the stars. One star sees a blink, and 1 second later the other star sees a blink. So the shadow (or the end of the light beam) can move 1/2 light year in one second. But this is not a way for the red star to tell the purple star anything in 1 second -- all that the purple people find out is that you decided to point a flashlight at them, a whole year ago.

My Question:"Why don't extended sources of light make sharp shadows?"
What is the difference between a point source of light and an extended source of light? How come when a point source of light is used to cast a shadow, the shadow is totally dark whereas with an extended source of light an umbra and penumbra result?
Sally's explanation
An extended source is like a frosted light bulb -- light comes from many places, and at any point in space there is light going different directions. A point source is a very small source, so that now there is just one point that the light comes from. Of course, all light sources are to some extent extended, though I have one made from a laser pointer for which the source certainly is very tiny. For many purposes, an unfrosted light bulb is a good approximation to a point source.
As you observe, point sources give sharp shadows, while extended sources give fuzzy-edged ones (that is already the distinction between them!). Here is a picture that explains how this comes about:
Light travels in straight lines from the source, in many directions. In the top picture the blue object is making a sharp-edged shadow on the screen, because either there is a path for light to take from the light source to the screen or there is not. I have indicated the region of (total) shadow by shading it, and indicated the part of the screen that is lit by coloring it red.
In the bottom picture I have drawn some lines from various parts of the light source. We observe that some parts of the screen are illuminated by all of the light source (colored in red again), some regions receive no light at all (shaded again), and some regions are illuminated by part of the light bulb but not all of it. I colored this part of the screen purple.
Here are pictures of a clear bulb (almost a point source) and a frosted bulb (an extended source), and shadows made by them. On the subsequent pages, you can see what a bug sitting on the screen would see, looking back at the bulb. This is another way of understanding why the extended source has a penumbra that is neither light nor dark -- these regions can only see part of the light bulb.

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My Question: "Can mirrors amplify light?"
Joe's answer Mirrors can bring light that would otherwise end up different places to the same place, making the light brighter at the chosen spot. For example, the Palomar and Hubble Space Telescopes use a big curved mirror to gather light from many square yards and concentrate it into a pin point. However, the total energy isn't changed, and some people would reserve the word "amplify" to mean actually increasing the amount of energy being delivered. The old-fashioned megaphone (just a cone with an opening to shout into) directed sound and made it louder -- it was a kind of mirror for sound. A public address system turns electrical energy into sound energy and it really is louder: that is amplification. So I would say that a mirror concentrates light but does not amplify it.
More of my question:
There is a story that Edison once used mirrors to "amplify" light from a lantern in order to fully illuminate a room at night. Is this possible?
Joe's comment Oooh, I didn't think of that! Certainly we can redirect all the light that was going to go to waste on the walls and ceilings to the place we want it, and produce a patch that is almost as bright as the light as it comes out of the lantern. Painting your room walls white makes it better lit -- it doesn't amplify the light but it recycles it a couple of times. The room light is brighter because the light stays around longer. Mirrors all over the place is the same idea, somewhat improved (getting more lanterns would accomplish the same thing and be a lot simpler -- but that wouldn't produce a story to tell).
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My Question: "Do shadows on the moon have a color ?"
Sally comments I haven't been there... but I suspect (and recall being told) not much. The color of a shadow is determined by light arriving in the shadowed region. On earth, the sky scatters some of the blue light from the sun, and then shadows outdoors may be slightly blue. Trees scatter green light, and so you may have a green shadow if you are standing near a tree. On the moon, there is no atmosphere and no trees and no clouds, and the rocks themselves are actually quite dark, so that very little of the sun light is scattered by them. So shadows would be very dark compared to the regions where the sunlight reaches... except on the side of the moon that faces the earth! The earth is a much bigger object in the moon's sky than is the moon in ours. Since the moon always presents the same face to the earth, a person standing on the moon always sees the earth in the same place in the sky, while the sun slowly moves across the sky once per month. When the sun is high, the moon is looking at the dark side of the earth, but near lunar sunset, half of the earth is illuminated and so shadows will be earth color: blue again.
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My Question: "Why is moonlight cool?"
Joe explains
You can barely read newspaper headlines in full moonlight, even though your eyes adapt a lot to the brightness of the light. It turns out that moonlight is a million times less bright than sunlight, and so it is a million times less effective at warming you up.

If the moon were a perfect mirror, oriented to reflect all light towards the earth, moonlight would still be less bright than sunlight, because the earth is larger than the moon. But in fact the moon reflects the sunlight that hits in in all directions; only a little bit of the sun's light gets redirected towards earth. Finally, the moon absorbs most of the light that hits it (a moon rock is actually pretty nearly black).
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My Comment: "Taking a flash photograph through a window"
I was watching a friend record a song in a recording studio, and decided to take some flash photos through the glass. A lady in the studio told me she was a professional photographer and that I would not be able to take flash pictures through the glass. I remembered what I had learned about the way light reflects in the mirrors section of the Virtual Workshop on Light, and asked her if it would make any difference if I shot at an angle instead of straight on. She said it probably would not make any difference. I ignored her and angled my camera. I have the photos to prove this works. I also have some bad straight-on shots to prove that this does not work.
Joe comments This is a nice application of understanding how mirrors work. About 5% of the light hitting a glass surface is reflected back -- and 5% of the total output of the flash is a lot brighter than the scene you are trying to photograph. Putting your hand or a black paper shield to block all possible paths from flash to camera (reflecting from the glass) might be a good idea, too.
Here are two pictures I took through my front door. For one I was as close to the glass window as I could get, while for the other I stood back 8 inches.

In the second picture you can see the reflection of the flash, as well as the reflection of the house across the street.

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My Question: "Why do objects appear smaller..."
Why do objects appear to get smaller as you move away from them?
Sally's answer As shown in the picture below, the angle between top and bottom is smaller. When asked about size of an object, people try to use linear dimension (like inches) instead of angular dimension (like degrees). (People do not think in terms of angles). To do this, they have to project the scene onto a plane, like the yellow line in the picture. In effect, they are assuming the object is a certain distance away, as if this was a photograph of the scene, instead of the actual scene. Now the more distant object seems to be smaller (in inches).

A related example -- the moon seems smaller than my thumb, but when the moon is low in the sky, I discover that it is behind the trees across the valley and clearly larger than they are -- I have been forced to realize that the moon is farther away (and bigger) than I thought.
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My Question: "Size and shape of long shadows"
Late in the afternoon, my shadow looks really funny: it has great big feet and a tiny head. Why?
Joe's comment
Hey, it does, doesn't it! Here are two pictures I took of myself, just after sunrise. If you look at someone else's shadow, it will be very stretched, but the head is in the same proportion as the feet. But when you look at your own shadow, the head part is really far away, while your feet are just as far away as usual. So your head looks small, for the same reason that anything that is far away looks small (see Sally's explanation of that,
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My Question: "Size of the shadow of an airplane"
"Does the size of an airplane's shadow change as it gets higher?"
Joe's comment
I know it seems it ought to. It is usually true that as you move the shadow-making object closer to the light source, the shadow gets bigger. This is because the light rays travel in straight lines and are diverging from one another.

But for sun shadows the numbers don't work out: the point is that the sun is 93 million miles away, and you can only get a few miles closer to it in an airplane. To make the airplane shadow twice as large, it would have to be half-way to the sun.

The occasion on which I realized this was when I tried to estimate how big clouds are. I can see cloud shadows on a field from a hill top, or on a city from a tall building. The shadows are miles on a side. And that's how big the clouds are, too.
Shadows of clouds
I have watched the airplane shadow from the airplane window occasionally. As the plane is taking off or landing you can follow it for several minutes. As the plane gets higher, the shadow is farther away, and so it looks smaller and smaller. However, you can compare the size to that of barns and trucks on the highway; it's not changing size much. There's another problem with shadows made by the sun, which you will observe if you compare the shadow of the top of the flag pole to the shadow of the part near eye-level: the sun is not a dot on the sky, so that sunlight comes from many slightly different directions. This makes the shadow fuzzy. When the airplane is so high that it doesn't look much bigger than the sun (seen from the ground), its shadow will be extremely fuzzy and you can no longer see it.
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My Question: "I can't darken my classroom"
My classroom has windows without shades. How can I do the activities that need to be in the dark?
Sally's comment
This will be most serious for the shadow and light-beam activities. Here are some possible fixes:
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My Suggestion: "Flashlights as light sources"
I only have two electrical outlets in my classroom, and running wires for all the lamps is a mess (and it looks dangerous), so I got a half-dozen flashlights and used these instead. No wires! No red-hot light bulbs! Totally portable!
Joe's comment
This can be made to work -- there was a point at which we all but flipped a coin, deciding which way was better. Flashlights have some real advantages, but there are disadvantages, too: they need new batteries all the time; they aren't quite as bright; and the beam they make is not as good. The last point needs some more discussion: the silvery reflector thingy at the front of the flashlight sends a lot of light in the right direction, but the beam now comes from all over the front of the flashlight, and you will not get good shadows close to the flashlight, and you won't get a nice beam when you direct it at a slot cut in a screen. There are two ways to fix this: move the flashlight farther back, or put a cone of dark paper around the bulb (you have to take the flashlight apart to do this), so that the reflector is hidden. Either way, the flashlight will be still dimmer, but now all the light comes from a very tiny bulb, and you will get very good shadows indeed.
For the lens activities flashlights work very nicely. You can tape waxed paper (or anything else that is translucent) with a diagram drawn on it onto the front of the flashlight, and this gives you an object to make an image of, which is flat and of easily measured size.

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Teachers' Comment: "Now we understand a projection TV"
After doing the unit on color we figured out something. There's a pizza place we go to all the time, that has this huge screen tv. We could never figure out why they had those three lights shining on the screen from a projector. Now we know that the projector is adding lights to make the many different colors on the screen, and why they only need the three bright lights on the ceiling to do this. --Melanie Trowel and Dawn Kelly, Crawford Middle School
Joe adds
As Ms. Trowel and Ms. Kelly explain, the three projectors are sending the red, green, and blue components of the picture. Seen separately, they might look like this:

We had a similar kind of TV projector at the Department of Physics until just a few months ago. Now I wish I had a photograph of it in action. It had the drawback that the three lights had to be aligned perfectly, so that the three color images superimposed correctly. But ours kept getting out of line, and then black letters would have red and blue edges.
Return to directory Why is the diffraction grating spectrum symmetrical?
Why is the white light spectrum symmetrical when viewed through the diffraction grating, and not show red, orange, yellow,...,violet on both sides?
Joe's answer
Here is a photograph of what happens when you put a diffraction grating into a light beam. The viewpoint is looking down one of the spectra, so the picture is a little distorted, but what I see is a central white beam (light that went through the diffraction grating without being scattered) and then two spectra, one to the right and one to the left. On each side blue is on the inside and red is on the outside. And the question is, why is it symmetrical this way, instead of having the colors go the same way on both sides?

The answer is that the effect of the diffraction grating is due to a comparison of the wavelength of light with the spacing of an array of parallel grooves on the diffraction grating (the grooves are only 1/10000" wide, so you can't see them or feel them, but they really are there). Wavelengths that are small compared to 1/10000" are not deflected very much, while longer wavelengths are deflected more. Blue light is shorter wavelength than red, by almost a factor of two, and so it is deflected less than red. If light is going to be deflected to the right, it is also going to be deflected to the left (think what happens when you turn the diffraction grating over, so that the top becomes the bottom -- the old right becomes the new left). So we have the symmetry you observe.

Rainbows and prisms make their spectrum a completely different way, so that there is no symmetry -- a prism makes just one spectrum, and the second rainbow that you sometimes see is fainter and wider (the colors are in reverse order, but this is because this rainbow comes from an extra reflection inside the water drops).
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stars in a blue sky My question: If scattered sunlight makes the sky blue, why doesn't scattered moonlight make the sky blue, too?
Sally explains:
It is blue! But it isn't very bright. The moon is a million times less bright than the sun, and so the resulting blueing of the sky is a million times less bright, too. Our eyes just don't notice it. However, if you do a time exposure photograph, so that a moonlit scene looks as if it were day, the sky is blue -- with stars in it! Here is part of a photograph of a scene at full moon taken by
Michael Martin. He was photographing the moonbow at Cumberland Falls (more about that at his web site), which is the colored blur at the bottom; the sky is blue, and if you look carefully you will see the trails left by the brightest stars in this time-exposure photograph.

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My Question: Why does light cause colors of fabrics to fade?
Joe's answer
Dye molecules absorb some kinds of light and reflect others; this way we remove the red light from white light and reflect blue, giving a blue-looking object. To get bright colors, we need to absorb a lot of the red and none of the blue, and the chemicals that do this are not so very common. So we may have to settle for chemicals that are a little less sturdy than we would like (especially after we leave out the colorants that are poisonous. Mercury oxide is red, and copper sulfate is blue, and copper arsenate is green -- but we are not not not going to wear clothes with these chemicals in them).

So the short answer is, when the molecule absorbs light, it may undergo some kind of physical or chemical reaction and become a different molecule, and now it is a different color.

However, a longer and more interesting answer involves the way light delivers energy. It turns out that it comes in little lumps called photons or quanta, and that the quanta are bigger for blue light than for red light (it was specifically for this idea that Einstein won the Nobel Prize. You are asking The Right Question, except that you are 100 years too late on this one. But Einstein didn't get a Nobel Prize for his science fair project, either, so hang in there). Thinking of energy as being like money, a beam of light is a rain of red nickels and blue dimes, instead of a steady flow of money. The result is that blue light is more likely to cause physical and chemical change than red light. (Because the dyes are like a candy machine that will "absorb" a dime and give you a pack of gum but if you put a nickel in, nothing happens; if an ultraviolet quarter comes along it is sure to cause something to happen).

The most obvious example is that UV light causes sunburn. More subtly, luminous materials (the glow-in-the-dark stuff) are activated by blue light and not red light. Older kinds of black and white photographic film, the blueprint process, and the early photocopiers could not see red light (red lettering came out dark, blue lettering came out light, and that is why blue pencils were used to annotate drawings that were going to be blueprinted). And it is my observation that color photographs that have been put on display turn blue with age: the red dyes absorb the blue light and get killed, the blue dyes reflect the blue light and survive.

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My Question: What is a light beam made of?
Joe's answer
Light is a kind of radio wave. It is a disturbance that involves electric and magnet fields.

Electric and magnetic fields tend to be smooth curves -- they would really like to be straight lines. Violin strings and clotheslines also like to be straight. If you deflect a clothesline at one point by whacking it with a stick, you will create a wave that runs down the line. In the same way, if you make a kink in an electric field line, it will also run away. However, it runs really fast, and (in the case of visible light) the length of the "kink" is really short -- so we don't realize what we are looking at. 150 years ago, when the relationship between light and electricity and magnetism was first realized, the connection was hardly more than a metaphor -- light travels at the speed that the electrical theory predicts, and has some common features (for example, light can be polarized, and will be scattered in a color-specific way by something with a pattern of fine grooves on it, like a CD). But today we know how to generate radio waves that are much smaller than a millimeter, and we can generate and detect infrared light that has a wavelength that is much larger than visible light, to the point that to a physicist or an electrical engineer there is no real distinction between electromagnetic (radio) waves and light.

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My Question: How can I display beams of light in a dim room?
Sally's answer
You need a little bit of smoke or fog to scatter the light. If the background is dark, you might not need very much.

Here are some methods:
1. Actual fog, made by blowing warm wet air over something that is cold (a fan, a bucket of hot water, and some dry ice bricks is a specific method I have hear of being used). The advantage is that the fog is just water and disappears without a trace. The problems are that the fog may disappear immediately if the air in the room is dry (as it frequently is, in the winter), and you need all the machinery -- buckets, fans, dry ice.
2. Actual smoke. Burning incense or cigarettes might do it. The advantages are that it is pretty simple to do; the drawbacks are that it is dirty and smells up the place a bit. However, just making pancakes for breakfast makes the air in my house smoky enough to give a clearl light beam effect (the door has a pattern of glass panels on it, which give rise to the light beams that the smoke reveals). You will note that the dark background is important to making the light beams visible.
3. Manufactured fog. There are devices that produce an oil fog, used occasionally in dramatic presentations. I believe these can be rented. The oil keeps the fog from evaporating too fast. I don't think very much oil is actually involved, but I guess it will wind up on the walls and windows.
4. For a small display, an aerosol can full of airfreshener might make enough of a cloud, though I don't know how long it would last. A theatrical materials or magician's supply shop might have something designed for the purpose.
5. Just running a vacuum sweeper for a while might make the air dusty enough!

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My Question: How can I make a curved mirror?
Sally's answer
This depends a lot on what you want to use the mirror for.
*Curved mirrors are used in telescopes, for example, and these must be extremely precise and the surface must be very highly reflecting. Amateurs make them all the time, but it is the sort of project that you work at all weekend for months.
*A curved mirror can be used for a magnifier. You probably already have one around the house -- "make-up mirror" is probably what it is called. These are not as fussy about precision. In fact, a metal spoon with a reflecting finish will already show you the effect; it might be fun to see how many examples you can find of curved surfaces that can be used as a magifier (or demagnifier, if you look at the other side).
*A curved mirror can be used to collect light, for a solar collector. I'm betting this is what you are trying to do. The good news is that this isn't very hard, because it doesn't matter too much if some of the light goes the wrong place.

The basic idea is to make a bowl that is pretty nearly the shape of part of a sphere, and then make the inside surface reflecting. This shape will reflect the sun's rays so that they all come together at (nearly) the same point. We want a wide mirror, so that it catches lots of sunlight; but making the wirror wider than my picture indicates (the part below the purple line) doesn't gain much and adds a lot of effort in the construction. The focal point is half-way between the center of the sphere and the sphere surface. I made a mirror of this sort once that was 4 feet wide. I was interested in reflecting sound, rather than light, which meant that the surface didn't have to be silvered. The easiest way to make this is to find a large round ball that is pretty round and smooth. A soccer ball or a beach ball might do. Hopefully, this is pretty well inflated, so that it isn't squashy and sagging. We can capture the shape of the ball by gluing many layers of paper strips to the ball (to make a paper mache' bowl). When the glue has dried, peel the bowl off of the ball, and then glue strips of aluminum foil to the inside.

Try out the solar collector by facing it towards the sun (in my picture I assumed the sun is straight up, which it almost never is); this should produce a bright (and hot) spot where the sun's rays meet.

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