21-2-einstein-and-the-photoelectric-effect_summary

This section is about Albert Einstein's explanation of the photoelectric effect, a phenomenon where light strikes certain materials and ejects electrons. This effect cannot be explained by classical physics and requires the concept of photons, quantum packets of energy, to understand. Photoelectric effect has several applications, such as in light meters for cameras, solar cells, and in the photoelectric devices used in automatic streetlights, elevator doors, and water faucets. In the movie industry, it's used to convert light variations on the film strip into sound in movie projectors. Einstein found that the energy of a photon is directly proportional to its frequency and inversely proportional to the wavelength, as given by the equation E = hf, where h is Planck's constant. The maximum kinetic energy of ejected electrons equals the photon energy minus the binding energy of the electrons in the specific material. The photoelectric effect has five important properties: 1. Below a threshold frequency, no electrons are ejected, regardless of the light intensity. 2. Electrons are ejected without delay once the light hits the material. 3. The number of electrons ejected per unit time is proportional to the intensity of the light and is independent of other characteristics. 4. The maximum kinetic energy of ejected electrons is independent of the light intensity. 5. The kinetic energy of ejected electrons equals the photon energy minus the binding energy of the electrons in the specific material, as given by KE = hf - BE. In quantum mechanics, the photoelectric effect is strong evidence for the existence of photons. The properties of the photoelectric effect cannot be explained by the wave model of light and are only consistent with the particle model, where light is composed of photons.