Photoelectric Effect

When light of a particular wavelength and frequency strikes a metal, electrons are ejected from the metal. This phenomenon is known as the photoelectric effect.

However, only light above a threshold frequency can eject electrons from the metal. Lower frequency light does not eject electrons, regardless of its intensity. How is this so?

Albert Einstein proposed that light behaves as a stream of particles or tiny packets. One packet — or quantum — of light is called a photon.

Each photon has an energy, E, that depends on its frequency, ν. The two are related by this equation, where h is Planck’s constant and has a value of 6.626 × 10⁻³⁴ J⋅s. Light, and thus photons, of higher frequency have greater energy.

An electron is bound to a metal with a binding energy, phi. This is also known as the work function of the metal, W. When a photon with energy greater than phi strikes the metal, its excess energy is transferred to the electron as kinetic energy and the electron is ejected. Thus, the kinetic energy of the electron is the difference between the energy of the photon and the binding energy of the electron.

To summarize the photoelectric effect, low-frequency, long-wavelength light does not eject any electrons from the metal. Increasing the intensity of low-frequency light only increases the number of photons. Each photon still has the same energy, which is not enough to dislodge any electrons.

Shorter wavelength light with a frequency higher than the threshold frequency does eject electrons because each photon is higher energy. As the frequency of light increases, so does the energy of the photon, and thus, the kinetic energy of the ejected electron.

The greater the intensity of light, the more photons strike the metal, and the greater the number of electrons that are ejected.

The photoelectric effect demonstrates the particle behavior of light.