The electrons are emitted when the photons of the incident light are able to transfer enough energy to the electrons to break them free from both their atomic bonds and the forces of the entire maternal lattice Materials in which this phenomenon takes place easily enough to produce many electrons when struck by visible light are called photoeminive materials Because the photoemissive material is usually housed in a glass tube, photoemissive devices are also often known as phototubes.
Figure 1431 shows the basic principle behind the operation of phototubes The surface of a specially shaped cathode is coated with a phot ve materi- al(such as cesium-antimony) The cathode(now called a photocathode) is housed in a sealed glass tube along with another electrode called the anode.
A voltage is created between the photocathode and the anode(with the anode having the positive voltage level).