Photoelectric effect

When light shines on a metal surface it emits electrons. 

 This is due to the fact that when a photon of light incidents on the surface of metal (and hits an electron in an atom), it contains enough energy to knock the electron out of the atom.

In other words photoelectric effect is a phenomenon in which, when the light(photon) of appropriate wavelength or frequency incidents on the metallic surface called photo cathode, then due to the production of photo electrons, photocurrent is generated.

 In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function (the electron binding energy) of the material, it is ejected. If the photon energy is too low, the electron is unable to escape the material. Increasing the intensity of the light beam increases the number of photons in the light beam, and thus increases the number of electrons excited, but does not increase the energy that each electron possesses. The energy of the emitted electrons does not depend on the intensity of the incoming light, but only on the energy or frequency of the individual photons. It is an interaction between the incident photon and the outermost electron.