Particle Nature of Light (Photoelectric Effect)

When light falls on mettle, the electrons may be ejected from the surface of the mettle. These electrons are called photoelectrons. Let’s perform an experiment with an emitter E and a collector C in vacuum. A potential difference V is established between the emitter and the collector in this way we can eject electrons from the surface of the emitter E and can collect them at C. These electrons produced electric current we can measure this current as photoelectric current i in the external circuit.

 Einstein said that the photons of light strike with the electrons of the mettle. The energy of the photons transmitted to the electrons, increasing their kinetic energy and causing them to eject from the surface of the mettle. Ejection of electrons does not depend upon the intensity of light rather it depends upon the frequency of the light. Photons with the frequency higher than a particular frequency can eject an electron from the mettle (emitter in this case). This particular frequency is called “Threshold Frequency”. The relation between frequency and energy is directly proportional means more frequency, more energy, and vice versa.

                                                           E = hv

Where E is the energy and v is the frequency while h is the planks constant and its value is 6.626 x 1034. Therefore higher frequency results in the production of high energy photoelectrons. So, the kinetic energy possessed by photoelectrons bears a linear relation with the frequency of the incident light.

K.E = hv –hv₀

Here hv is the energy of the photon and hv₀ is the Threshold frequency below which no emission of electrons occurs and h is the planks constant. hv₀ is also called the work function and the equation can be written as:

hv = w + K.E

           Now back to our example, this equation shows that a single photon carries energy to the emitter and transferred it to the electron. A part of this energy called the work function of the emitter that used up in causing the electron escape from the emitter’s surface and remaining appears as the kinetic energy of the electron. This kinetic energy as mentioned is due to the frequency of the incident light. Then what about the intensity of light? The intensity of light refers to the luminosity of the light or how many photons strike the emitter surface. More intensity means more photons strike the electrons and the rate of emission of electrons will increase but the kinetic energy of each electron will remain the same. It means the frequency of the light increases the kinetic energy of the electrons and does not affect the emission rate while the intensity of the incident light increases the emission and does not have any effect on the kinetic energy of the electron. This kinetic energy of the electrons can be measured if we connect the emitter to the positive of the battery and collector to the negative end, providing a large potential difference we eventually reach the point where even the more energetic electrons turn back before they strike the collector and the photoelectric current i becomes zero. This potential is called the stopping potential V₀ and the stopping potential is equal to the kinetic energy of the photoelectron.

           From the above discussion, we can deduce the following facts:

• Stopping potential does not depend upon the intensity of the incident light. The frequency of the falling light must be greater than a certain value of frequency called the Threshold frequency. Otherwise, the photoelectric effect will not occur.

• The emission of photoelectrons depends upon the frequency of the incident light, not on the intensity of light.

• Electrons are emitted as soon as light reaches the surface of the emitter. 

The above explanation cannot be explained by the electromagnetic theory if radiations but the quantum theory of radiations give its explanation.