Explain the photoelectric effect.

The photoelectric effect is a phenomenon in which electrons are emitted from a material's surface when light is incident upon it. This effect occurs due to the interaction between light (as photons) and electrons in the material, where a photon transfers its energy to an electron, causing it to be ejected if the energy is sufficient. Important observations include the direct proportionality between the number of emitted electrons and light intensity, the presence of a threshold frequency below which no electrons are emitted, and the dependency of emitted electron kinetic energy on light frequency, not intensity. Factors affecting the photoelectric effect include the material, frequency and intensity of incident light, and the angle of incidence.