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A phototube is a device
that responds to light by producing an electric current. Lifting
the cover off the tube reveals a curved metal plate inside. This
metal plate is coated with a material (Cesium and Tin) which emits
electrons (negative electric charges) when exposed to light. The
free electrons move from the plate to the small wire in the center
of the tube. This flow of electrons is an electric current, registered
on the current meter. This phenomenon—light causing metal to
emit electricity—is called the photoelectric effect. The photoelectric
effect is sensitive to color; it only works with light that has
enough energy to free electrons from a metal. In this exhibit, red
light doesn't produce a photoelectric current but yellow and green
light do. That's because low frequency red light carries less energy
than higher frequency yellow and green light.
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