Explain the photo-electric effect and describe how the intensity of light effects rate of electron emission.

The photo-electric effect is the way in which electrons are released from the surface of a metal when there is an incident light ray. The particle model of light, as part of wave-particle duality, suggests that light is made up of discrete packets of energy called photons, these photons have energy equal to the frequency of the light rays multiplied by the Planck constant. When the incident light rays strike the metal surface there are 1 to 1 interactions of photons and electrons in which surface electrons absorb the incident photon and become excited and can move to higher energy levels. Should the energy of the incident photons be above the work function of the material the electrons gain enough energy to overcome the attraction of surrounding nuclei and escape the metal being released as free photo-electrons. Intensity is an important factor to consider in for this effect, this intensity of a light ray indicates the photon number density in the ray hence rays of higher intensity contain more photons and, as interactions between photons and electrons are 1 to 1, increasing the intensity of light will increase the rate of photo-electron emission. It is important to note that the intensity of light is irrelevant in determining if electrons are released as if the frequency of the light is below the threshold frequency then photon energy is below the material's work function and no electrons can be released irrespective of ray intensity.