Photo Electric Effect

 

Photo electric effect

             When ultraviolet light of crtain frequency falls on a metallic plate then som electron emits from the surface of metal which are called photo electrons. Current is called photo current while the phenomenon is called photo electric effect.

Working: When ultraviolet light falls on metallic surface plate then photo electrons emits from it and moves towards +ve plate (collector) and galvanometer shows deflction due to photo current.

Apparatus setup

For maximum energy of photo electrons: to find the maximum energy of photo electrons we change the terminals of battery now plate becomes +ve and collector becomes –ve.

When ultra violet light falls on metallic plate then electrons emitting from the surface of metal but due to +ve charge on plate some electrons deposit on it which are called captured electrons and those electrons reaches to the –ve collector whose energy is maximum. If we make collector more –ve ontinously then after sometime no no electrons reaches to collector and galvanometer shows zero deflection current is called stopping potential denoted by V_o

                                                           (K.E)_max = V_oe

1/2mv_o² = V_oe

Here,                       v_o is maximum velocity

Hertz observation

1.      For the emission of photo electrons from the surface of metal a light of certain frequency is reuired this minimum frequency is called threshold frequency  denoted by υ_ο it means that no photo electron emits from metallic surface if the of incident light is less than threshold frequency.

After threshold frequency

2.      The energy of emitting electron is directly proportional to the frequency of incident light.The number of photo electrons emits from metallic surface are directly proportional to the intensity of the incident light.

Graphical result

1.      Graph between frequency and kinetic energy of photo electrons: when the graph plotted between frequency of light and kinetic energy of electrons, then it shows that no photo electrons are emitting when frequency of incidentlight is less then a certain minimum value which is called threshold frequency, after threshold frequency the kinetic energy of photo electrons is directly proportional to the frequency of light.

2.      Graph between current and potential: (a). When frequency is constant but intensity is different:  when the graph is plotted between current and potential at same frequency but different intensity of incident light then it shows that at a particular potential ( v = 0), current becomes constant which is called situration current ‘I_o’, if we decreasing the potential towards –ve side at a particular –ve potential current of all different intense lights becomes zero, this potential is called stopping potential. Situration current depends upon intensity of incident light but stopping potential is intensity independent. It means that energy of photo electrons does not depends upon intensity of light.

(b). When frequency different but intensity constant: When the graph is plotted between potential and current at same intensity but different frequency then the stopping potential is frequency dependent but saturation current is frequency independent, the energy of photo electron depends upon frequency of of incident light.

Some important definitions

1.    Threshold frequency: The minimum frequency of incident light which is required to enjection of photo electron from the metallic surface is called threshold frequency denoted by υ_ο.

2.    Work function: The minimum energy of incident light which is required for the enjection photo electron from the surface of  metal is called work function denoted by φ_ο and its value depends upon the nature of material of plate.

                                              φ_ο = hυ_o

                                                             φ_ο = hc/λ_ο                                              λ_ο = cut off wavelength

3.    Stopping potential:  The potential across the metallic plates (collector and plate) at which photo current becomes zero is called stopping potential denoted by V_o.

Einstein’s photoelectric equation

               According to quantum theory, light consists large number of packet or bundle of energies which are called Quanta or Photon. When a photon of energy (E = hυ) falls on the surface of metallic plate then some packets of its energy utilize to eject the electron from surface of metal and remaining part used in kinetic energy of photo electron, mathematically it can be written as.

Failure of classical physics

1.      There is no concept of threshold frequency.

2.      Wave or light can not eject the particle (electron) from surface of metal.

3.      According to  classical physics energy of photo electrons depends upon intensity (amplitude) but experiment shows that the energy of photo electrons depends upon the frequency of incident light.

Construction: For the production of photo electrons a glasstube which consists two metallic plates one is connected to positive terminal of the battery, called collector and other is connected to negative terminal of the battery called plate. A galvanometer is also connected in series to indicate the photo current or flow of photo electrons.