21.2 Einstein and the Photoelectric Effect

• When light strikes certain materials, it can eject electrons from them. This is called the photoelectric effect, meaning that light (photo) produces electricity.
• One common use of the photoelectric effect is in light meters, such as those that adjust the automatic iris in various types of cameras.
• It means that the apparently continuous stream of energy in an EM wave is actually not a continuous stream at all. In fact, the EM wave itself is actually composed of tiny quantum packets of energy called photons.
• In equation form, Einstein found the energy of a photon or photoelectron 
• If you have watched streetlights turn on automatically in response to the setting sun, stopped elevator doors from closing simply by putting your hands between them, or turned on a water faucet by sliding your hands near it, you are familiar with the electric eye, a name given to a group of devices that use the photoelectric effect for detection
• The photoelectric effect is the process in which EM radiation ejects electrons from a material.
• Einstein proposed photons to be quanta of EM radiation having energy 𝐸=ℎ𝑓,E=hf, where f is the frequency of the radiation.
• All EM radiation is composed of photons. As Einstein explained, all characteristics of the photoelectric effect are due to the interaction of individual photons with individual electrons.
• The maximum kinetic energy KEe of ejected electrons (photoelectrons) is given by 𝐾𝐸𝑒=ℎ𝑓−𝐵��,KEe=hf−BE, where hf is the photon energy and BE is the binding energy (or work function) of the electron in the particular material.