Looking around the house, I found a small collection of lenses: one was the lens I had used in the eyeball, from a big magnifying glass; one was my wife's reading glasses (about +1.25 diopters), one small magnifying glass, and another pair of glasses, about -8 diopters. In addition I had a big fresnel lens (like the kind you can glue to the back window of your van). I taped a small box to the blackboard (see the picture above), and for each of these lenses I had made a cradle (out of 3 layers of cardboard of course), which I could stick onto the end of the box with a pair of roofing nails such that half of the lens sticks out. The same shop light I used before was set up on the other end.

First I mounted the big lens on the box, to show what effect we were after.All incoming light gets bent towards one point, the focal point. When you run your finger through the light, the shadow on the far side of the lens pivots through the focal point. The same thing can be shown using a little laser pointer. To explain how this happens, I replaced the lens with the prism. You can see the light being bent down. The lens from the magnifying glass is fat enough to show that you could imagine it being made up of prism-like shapes.

With these toys to play with, it is easy to show the 3 simple rules of (positive) lenses:

1. A ray of light going through the center of a lens is not bent. After all, the middle of a lens is almost like a flat piece of glass.
2. A ray coming in horizontally, goes out through the focal point on the other side.
3. A ray that comes in through the focal point on one side, goes out horizontally on the other side. (this is the same as turning the rays from rule 2) around).

Finally, I showed that you can use one lens to make an image of a distant object, and then use another lens as a magnifying glass to look at that image: now you have a telescope. Any two lenses can do this trick. I used my wife's reading glasses and a small lens (plus some cardboard tubes and tape) to make a telescope. We looked at leaves and branches of faraway trees.

• Optics (Lenses)
• The overhead projector explained

What is wrong with is picture, OR, what shape is that christmas ornament?

The first thing to get across is that you can add waves up. If you stand in a quiet surf up to your knees, and the water rising and falling as the waves roll in, you can drop a stone in the water making smaller waves on top of the big ones: you can add waves. I had drawn waves on big strips of paper (5'x8") that I could tape to the blackboard.
Using two identical paper strip waves (wavelength 20 cm), I lined them up, and on the board drew the result of the addition, a stronger wave with the same wavelength. The do the same thing with the waves out of phase, and the result is no wave at all.
Then I replaced one of the waves with another one with a longer wavelength (23 cm), and we added them up. The result is a beating pattern, where the wave gets stronger and weaker.
Time for the first demo: I had bought at Radio shack two identical simple beepers (I got this from John Seagrave, who also does science demos in Santa Fe), and one 9V battery. Hook one up - you get a single continuous shrill note. Hook up the other one - same note. However, when you hook both up at the same time, the inevitable tiny difference in frequency between the two leads to a very strong beating of the tone, a sort of warbling sound. You can easily influence the frequencies by small movements or by holding something nearby, and the beating will get faster or slower.

Next we moved to light waves. I had set up to make vertical soap films. About a quart of bubble juice (10:1:2 water:diswashing liquid:glycerine) in a cookie sheet, and a coat hanger bent into an oval (so it would fit into the cookie sheet). It really pays to get the right background and lighting. I had a small wire hook taped to the blackboard to hang the coathanger loop from. Get a black background (like a sheet of of newspaper tempera-painted black - you have to improvise when you do this late the night before). For light I used a shop light clamped onto my tripod, with a big sheet of white paper draped in front. Anyway, the colors are spectacular, as are the swirls and movements in the soap film.
Horizontal color bands form quickly, pink and green near the bottom, higher up red, yellow, then white and 'black' or invisible. Then the film pops and you do it again. The picture at the top of this section is of them, and here are some more pictures of soap films.
Where do these colors come from?

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• From the Exploratorium: make an observatory to see colors in bubbles
• Graphics Archive - 120 Cell Soap Bubble by John Sullivan
• Bubbles on Brian Carusella's site.
• Professor Bubbles
• Pretty bubble pictures

• Modulation
• Waves
• Modulation