Describe the photoelectric effect and what it tells us about the properties of light .

The photoelectric effect describes the process by which surface electrons are emitted from a metal when light is shined on it. This occurs because the electrons are able to absorb the electromagnetic energy and escape the surface of the metal.

During the photoelectric effect we observe that electrons will not be emitted from the surface of the metal if the light intensity is increased unless the incident light is above a certain frequency known as the threshold frequency. Above this frequency, increasing the intensity of the light increases the number of electrons emitted.  This tells us that light at a higher intensity must contain more quanta of energy, known as photons. These photons collide with the electrons and excite them out of the metal's surface.

Increasing the frequency of the light above the threshold frequency increases the maximum kinetic energy of the electrons released, proving that at a higher frequency each photon has more energy (Energy = Planck's constant * Frequency). Below the threshold frequency, the photons don't have enough energy to excite the electrons off the surface of the metal. This effect therefore illustrates light behaving as a particle, because if it was a wave, increasing the intensity of the light would result in electron emission, but it does not.