Photo electric effect: When some photosensitive substances are exposed to ultraviolet light, electrons are ejected from their surface which are called photoelectrons. This process is called photoelectric emission.Laws of photoelectric emission:(i) There is a definite cut off value of frequency below which electrons cannot be ejected by any substance.(ii) Number of emitted electrons are directly proportional to the intensity of light incident.(iii) Kinetic energy of emitted electrons depends on the frequency of incident light on substance.(iv) There is no time logging between the incident of light and emission of electrons.