The free electrons on the surface of the metal body can be emitted by applying light on that surface. When a beam of light strikes on certain metals like potassium, sodium and caesium, electrons can get emitted from those metals. As per quantum physics, light is nothing but a stream of photons. Photons are similar to packets of energy. The energy of a photon the product electromagnetic frequency and Planks constant. When e is the energy of the photon, h is the Plank's constant and λ is the electromagnetic frequency of the incident light. During collision of photons with the free electrons that photon energy gets transferred to the electrons.
Although the energy of all photons cannot get transferred to the free electrons, the energy of photons which got transferred to the free electrons may be greater than the work function of the metal. The transferred photon energy to the free electrons when it is greater than work function can cause the electrons to get knocked out from the metal surface. Hence there is an electron emission due to applied light on the metal surface. This process of electron emission is referred to as photoelectric emission. Here the emitted electrons are called photoelectrons. The amount of photoelectric emission depends upon two factors.
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One is the frequency of incident light. Other is the intensity of light, i. e. number of photons in the beam of light. If frequency and intensity of the incident light increase, the amount of photoelectric emission gets also increased. The photoelectric emission is used in phototube. The cathode and anode of a photoelectric emission system are enclosed in an evacuated transparent glass container. The anode A is at a positive potential in respect of cathode or emitter. This potential difference is created by a battery. When the light of a required intensity and frequency strikes on the emitter surface electrons get emitted and the emitted electrons are attracted towards the anode and cause a current flowing through the external circuit which can also be indicated by a galvanometer connected to the external circuit.