Laser Diffraction Demonstration

Laser Diffraction Demonstration

Hi there!

Here is a quick and dirty overview of a demonstration I had in mind, using photos to show the coolness! Also they show that I'm still here at 8:40pm...

First, give the kids these cool glasses. I'm sure that'll take a few minutes. I have about 50 of them.

They make everything look like crazy rainbows! That'll probably take a few minutes more with the students.

To take the above photo I just put the glasses over the lens and sparked a lighter. Any point source of light looks really cool through them, the point source is split into all the colors that are composing it.  Looking at a strand of multi-colored christmas lights (not pictured) you can see the spectrum coming from say, a red bulb, isn't pure red, but contains some other colors that aren't being seen unless they are spread out in a spectrum through the glasses. You can also see the same thing for a blue bulb, but what is coolest is comparing the rainbow spectrums and noticing that the blue bulb spectrum is strong in the blue and the red is strongly red... they look like different types of rainbows I guess... hard to describe. The glasses are two layers of diffraction grating at 13,500 lines of dark, clear, dark, clear etc. per inch. It is cool they look so clear, but the grating is very small, so that's why it works. 

At this point would probably be some talking to the kids, explaining about the glasses, then I'd like to touch on how/why light can be interpreted as a wave. I would point out to the class that light obviously isn't a wave, it's color, right? Look at this green laser, it's not a wave, it's green!

But that doesn't have a lot of utility for explaining some of the things light does. I would then shine the green laser through the glasses, showing the effect:

The green laser light is projected as a grid! Why? Before the glasses just spread out all the colors into a rainbow... why isn't the green all spread out like that? Why are there dark spaces in between the green dots?

This would probably be a good time to ask the class if they can think of anything else they've ever seen that behaves like this, what gets spread out after it squeezes through a small space? I would say it's almost like the light is "splashing" as it goes through the small grating of the lens. Then maybe we would talk about the difference between saying light is a wave and saying light behaves like a wave.

Then we could talk about what different kinds of waves there are, really big long ones, really short fast ones. Then I'd claim that maybe different colors of light behave like different size waves, and ask the class to make a prediction about whether bigger waves will splash farther apart or not as far apart as a smaller wave through the same size opening. Hopefully it will be easy to show that longer waves splash farther. 

I could ask a student to guess, if light is a wave, and different colors are different size waves, if you had to guess, which colors are "longer" waves than others? What is it about red or blue or green that could clue you in to whether it was a bigger wave or not? Perhaps some kid will come up with a good reason, but I can't think of one, I'd like to bring it up to point out how some aspects of nature are hidden from us through normal experience.

Then it would be experiment time to prove our guesses! I would set up a little stand I have to hold the glasses and shine the green laser through to the board and have a student use a green marker to mark the location of the dots. Then we would shine a red laser through the lens up at the board and have a student mark the locations with a red marker.

It's a little dimmer, but I'm sure lining up the middle 9 would be good enough to show that the red laser is being spread out farther than the green laser. Then we could point out that if you say longer waves splash farther this is the only reason to say red is a "longer wavelength" than green, because of its behavior, not something about "red" as an experience itself.

At this point the demonstration would be over. If the kids were up for it, we could measure the spread of the dots on the board, and knowing the distance from the laser to the board and the width of the lens grating, we could calculate the relative wavelengths of red to green light. The equation would be simple algebra, just some division.