Photo electric effect & Einstein photo electric equation

Photo electric effect -

The emission of electrons from a metal plate when illuminated by light of suitable frequency or wavelength is called photoelectric effect. The emitted electrons are called photo-electrons.

click here to know about photo electric cell

Explanation 

It consist of two photosensitive surfaces A & B in a transparent tube. Plate A is connected to negative terminal of the battery while Plate B is connected to positive terminal through galvanometer.

In the absence of light there is no flow of current. Hence there is no deflection in the galvanometer. When a light of suitable frequency or wavelength falls on plate

A, electrons are ejected out & attracted by the Plate B. Current starts flowing in the circuit indicated by the galvanometer.

like our facebook page for latest update

Characteristics of Photoelectric effect

1. Photoelectric effect is not possible below the threshold frequency.
2. The number of photo-electrons emitted is directly proportional to the intensity of incident light.
3. The kinetic energy of emitted electrons is directly proportional to the frequency of incident light.
4.  Emission of photo-electrons from cathode takes place instantaneously.

Stopping potential

The negative potential of anode for which the photoelectric current becomes zero is called stopping potential.

Threshold frequency - The minimum frequency of incident light upon which photoelectric effect occurs is called 'threshold frequency". It denoted by 𝜈ₒ. 

Work function - It is the amount of photon energy which brings the electron from metal's inner part to its outer surface. It is denoted by wₒ.

                                           E = wₒ + K.E

                                           wₒ = E - K.E

Einstein photo-electron equation

Consider a photon incident on a photosensitive metal surface. The energy of photon is spent in two ways-

1. To emit electron from inner part to the metal surface. which is equal to the work function of the metal.
2. To imparting the maximum kinetic energy to the emitted photo-electron.

click here to know more about photon

                      h𝜈 = wₒ + K.Emax    

               or    K.Emax = h𝜈 - wₒ .................(i) 

If the incident photon is of threshold frequency 𝜈ₒ

            then   h𝜈ₒ = wₒ

from equation (i)

                      K.Emax = h𝜈 - h𝜈ₒ

               or    K.Emax = h(𝜈 - 𝜈ₒ)

   we know     𝜈 = C/λ

            then   K.Emax = h(C/λ - C/λₒ)