Electricity & Magnetism


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.