Periscopes & Prisms

If you put a prism inside a periscope, will a rainbow come out the other end?

Yes, a student asked me this during Science Club one day.

And, yes, we tried it.

Because we had the perfect periscope to try out this experiment on!

Last week, we used the instructions in Spy Science by Jim Wiese to make a periscope out of two quart milk cartons.

Pro tip: Cut the top and bottom off the milk cartons as soon as they are empty and wash them out with some dish detergent, then dry them thoroughly and let them sit until you are ready to use them. Don't just swish some water in them and figure that'll be fine. Our cartons were a bit whiffy.

Use a pair of scissors, a ruler, a protractor (you want 45 degree angles), two small mirrors (ours were two inches square), some pieces of sturdy cardboard, and tape to make your empty milk cartons into a periscope!

Make sure that the slits you cut are parallel to each other, the mirrors are taped on facing each other, AND you have cut the viewing windows on OPPOSITE sides of the tube. The image comes in one window, bounces from one mirror to the other, and comes out the window on the opposite side.

After we made and tested these periscopes last week, the children started asking me tons of questions and suggesting other things we could try. The question about the prism was, I think, asked jokingly, and no one thought it would work. But science is about testing things even when you think you already know what will happen. And it got me thinking. As the light bounced from one mirror to the next, if it went through a prism, wouldn't it refract?

(By the way, I highly recommend this question, and this activity, for grade 6 Physics in the Waldorf curriculum: Sound, Light, Heat.)

So yesterday we tried it. Because our periscope was made of two cartons, it was very easy to cut the tape that held them together, tie a crystal to a string, hang it in the space between the cartons, and tape it back together.

the hanging crystal in the tube

the image of the hanging crystal in the other mirror

We knew this crystal worked well as a prism because ordinarily it hangs in my kitchen window. When the sunlight hits it at a certain angle, there are tiny rainbows all over my kitchen. The children love this and come running and calling to one another, "It's rainbow time!!!"

Once the periscope + prism was prepared, we found a place in the dining room where the sun was coming in and there was a very bright patch of direct light. Ms. Kamea did have to hold the periscope very carefully, to get just the right angle but, yes, it did cast a rainbow from the other end!

Many thanks to Ms. Kamea and Ms. Destinee for coming to lend a hand during our periscope explorations!!

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