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Q & A: Laser Light and Prisms

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Most recent answer: 05/17/2011
Q:
Why does the laser light seem to get caught in a prism while regular light from a flashlight goes out in all directions?
- Ev (age 9)
Texas,USA
A:
Ev -

The reason that this happens is because laser light is only one color of light, and light from your flashlight is all different colors of light. This probably sounds pretty weird, but let me explain. There are lots of different colors of light. Just like with different colors of paint, you can mix them together to get other colors. It's just that light colors mix differently than paint colors. If you mix all of the colors of light together, you get white light (like the light from your flashlight).

Prisms bend different colors different amounts. So when you shine a flashlight in, all of the different colors bend differently and get separated. (This is how you get a rainbow - by spreading the colors apart.) Lasers, on the other hand, are only one color. Some of them are only red and some of them are only green, but they're only that color. So when you shine a laser through a prism, there's nothing to be separated, and the light stays together. One thing that I think is pretty interesting to see is if you shine two different colors of lasers through a prism. Then the two laser beams spread apart from each other, but each one stays together with its own color.

Hope this answers your question!

-Tamara

(published on 10/22/2007)

Follow-Up #1: color sensing

Q:
Hello, as you say laser is single color, I have a little doubt. As if you take any color in this universe it's mixed with other colors to get one color out. Consider red; it is mixed with green and yellow. And green is mixed with 2 more and yellow is mixed with 2 more to form themselves and so on. I request your thoughts on this. Tabrez
- Tabrez (age 31)
India
A:
Laser light (from a typical laser) is indeed very nearly single-wavelength. Whatever that wavelength is, it triggers the three different color sensitive types of cells in your eye to different degrees. Your brain interprets the ratios of different signals from the three types as a visual color.

I think what you're pointing out is that you can use a mixture of different wavelengths and still get the same ratios of signals from the three cell types. Then your mind will see the same color even though the combination of wavelengths is different.

So when you see a nice blue-green from an argon laser, it's almost precisely at 488 nm wavelength. However, you can get the same color sensation with various mixed-wavelength combinations. It's easy to tell the difference with a prism, however. The laser light stays a single beam, but the mixed wavelengths split up into different color beams.

Mike W,

(published on 06/12/2008)

Follow-Up #2: reflecting specific colors

Q:
I had a question about this light bending. Since prism bend light sources with mixed light. Would it be possible to construct a prism that bends only a certain color out of that mixture of light? IE. I shine a flashlight into my special prism, and only green is seperated while the rest stay together? I know if I placed a specific tranquil cover on one side of the prism, it will only display that color on the otherside (Just as if i put green plastiv over my flashlight, now only the green light makes it through the plastic) but what I'm asking is if we can specifically seperate a specific color from light, while leaving the rest untouched? I would think it might be possible based on the idea of Sound waves (Which are different i understand). But if you filter sound enough, you can usually get the 1 sound you want out of an enviorment of noise. Am i just talking crazy now? :)
- Travis Fry (age 27)
Scottsdale, Az Maricopa
A:
You're exactly right that the wave behavior of light is enough like the wave behavior of sound that you can draw useful analogies.  

It turns out you can't make something that picks out a single specific wavelength and doesn't affect any others. Any real, finite, device always affects some range of wavelengths. However, by stacking many reflectors one can make a reflector that mainly affects only a narrow range of wavelengths, ones that look to us like just one color. The device relies on the wave interference between reflections from the different layers. You can buy these devices, called "interference filters", from optical supply companies. I've got a bunch of them in some drawer in the lab. A new half-inch diameter filter transmitting a roughly 10 nm bandwidth around 500 nm costs about $50.

Mike W.

(published on 06/04/2010)

Follow-Up #3: why doesn't laser light spread more?

Q:
Well I have a question. Lets say you have a blue laser and a blue night-light. If you turn them both on, the laser goes in a strait line but the night-light goes all over the place. Why?
- Chris (age 15)
A:
The easy part of your question is to describe the night light. It probably is a standard bulb (called "incandescent"), with a very hot little wire (called a "filament") in it. The wire is so hot that it emits light. The little particles in the wire that emit the light, the electrons, are moving every which-way, because they're so hot. There's no reason for them to choose one direction or another to emit the light. Bits of it go every possible direction.
 
Now we get to the hard part- the laser. It works in a rally different way. A collection of, say, argon atoms are prepared in high-energy states. When they fall down to lower energy states they emit the extra energy as light. So far, what I've described is not all that different from the high-energy electrons in the wire. However, there are key differences. The electrons are just plain hot, which means they have a whole range of different energies. The laser atoms are prepared in special states with very narrow ranges of energies. That means that each blip of light that they emit as they fall into their lowest-energy state has almost the same energy. Blips ("photons") with the same energy have the same color.

Ok, so finally here's an answer. Each atom in the laser would, on its own, take a fairly long time (by atom standards) to emit its light. However, the presence of the light from other atoms stimulates the emission of light in exactly the same state as the previous light. (That's a fundamental property of light emission, known since early work of Einstein.) So the new light is going in the same direction as the old light.

BTW the "se" in "laser" stands for "stimulated emission", the process we just described.

Now I've given you a reason why the laser light should beam in a single direction. How does the laser set the direction? Some partly reflecting mirrors in the laser allow light traveling along one direction to build up and trigger more light in that direction. Other directions don't build up.

I hope that at least gets you started.

Mike W.

(published on 05/17/2011)

Follow-up on this answer.