Photon Theory Of Light - Practice Questions & MCQ

Photon theory of light -

According to Eienstein's quantum theory light propagates in the bundles (packets or quanta) of energy, each bundle being called  a photon and possessing energy. The energy of one quantum is given by, hν , where h is the Planck's constant and ν is the frequency.

where c = Speed of light, h = Plank's constant = 

            = Frequency in Hz , = Wavelength of light. 

                                                                                                                    (In the form of eV)

Photon is a packet of energy (or) particles of light and travels with speed light in a straight line. Energy of photon depends on frequency and it doesnot change with change in medium. One more thing is that the Photons are electrically neutral and not effected by electoric and maginetic field. 

After energy now let us discuss the mass of the photon.

Mass of photon : 

You will study in the theory of relativity that the rest mass of any body is given by - 

As the velocity of photon is same as speed of light, so from the above equation we can write that -   .  But it's effective mass is given as - 

It is also called as kinetic mass of the photon.

Momentum of the photon - 

As the momentum of any body is  = 

Here the velocity = c,  i.e., speed of light. So, we can write that -

Note -

1. In a photon particle collision, total energy and total momentum will be conserved but the number of photons may be changed. 

2. All photons of light of a particular freq. (or) wavelengths have the same energy and momentum whatever may be the intensity.