70. Infrared, Radiometer, and Maxwell's Spectrum

You can show the rainbow spectrum of visible light with a dispersing prism in front of a slide projector. A transparency of Maxwell's Rainbow, next page, is available.

An infrared detector connected to a meter visible to the class is placed beyond the red in the spectrum above. Why is the signal so small? Doesn't the tungsten lamp peak in the infrared? Ah, remove the "heat" absorbing glass element from the projector and the signal becomes large.

Ultraviolet light and the fact that it is absorbed by window glass can be demonstrated in connection with the photoelectric effect.

A radiometer consists of a spinner in a near vacuum with vanes silvered on one side and blackened on the other. When light, particularly infrared, falls on the radiometer, the black sides of the vanes become hotter and drive residual air molecules away from them, propelling the vanes around. (The rotation direction is contrary to that produced by radiation pressure, a smaller force.) A flashlight will operate the radiometer, but a match flame, which has more infrared, does much better. (Low powered lasers, of course, have too little energy to operate the radiometer.) Parabolic reflectors can be used to focus the infrared from a match or electric coil to spin the radiometer across the lecture hall.