Photo Electric Effect

The effect of emission of electrons from the surface atoms of the metal plate when it is exposed to ultraviolet light radiations ,is called as photoelectric effect. The emitted electrons are called as photoelectrons and the resulting current due to them is called photoelectric current or photocurrent. Laws of Photoelectric effect:1. There exits the critical or particular value of the frequency of incident light radiation just at an above which the photoelectric emission is observed from the given metal plate, is called as the "Threshold frequency" ν_o . It is different for different metals and is given by

  Where, λ_o is the longest wavelength which is sufficient just to cause photoelectric emission. Below the threshold frequency, there is no photoelectric emission from the given metal plate, for any less or more intensity of incident light. 2.If  ν > ν_o then the maximum K.E of the emitted photoelectrons is directly proportional to the frequency of incident light radiation, but is independent of the intensity. 3. The number of the photo-electrons emitted per second or the strength Of photo-electric current is directly proportional to the intensity of the incident light radiation but it is independent of frequency. 4. There is no time loss or time lag in the photoelectric emission or photoelectric emission is an instantaneous process. Einstein's equation of photoelectric effect: Einstein assumed that light radiation consists of photons each of energy "hν"  Where h -planks constant ν-frequency  When the light radiation falls on the metal plate, there will be a perfect elastic collision between electrons of surface atoms of the metal plate and the photons. However during each photon-electron collision, the energy 'he' of each photon is completely absorbed by the electron. The absorbed energy 'he' is used for two works namely, a. The minimum amount of energy required just to free or separate the photoelectron from the surface atoms is called as work function (W_∞) .This is different for different metals. b. The remaining amount of energy is used in giving maximum K.E. (1/2 mV2max) to the emitted photo-electrons. Hence by the principle of conservation of energy, we can write,hν = W_∞+ ½mv²_max  OR  ½mv²_max = hν - W_∞.