The rest mass of the photon is

Assertion : The relative velocity of two photons travelling in opposite directions is C. Reason : The rest mass of a photon is zero .

Einstein established the idea of photons on the basis of Planck's quantum theory. According to his idea, the light of frequency f or wavelength lamda is infact a stream of photons. The rest mass of each photon is zero and velocity is equal to the velocity of light (c) = 3 xx 10^(8) m.s^(-1) . Energy, E = hf, where h = Planck's constant = 6.625 xx 10^(-34)J.s . Each photon has a momentum p = (hf)/(c) , although its rest mass is zero. The number of photons increase when the intensity of incident light increases and vice-versa. On the other hand, according to de Broglie any stream of moving particles may be represented by progressive waves. The wavelength of the wave (de Broglie wavelength) is lamda = (h)/(p) , where p is the momentum of the particle. When a particle having charge e is accelerated with a potential difference of V, the kinetic energy gained by the particle is K= eV. Thus as the applied potential difference is increased, the kinetic energy of the particle and hence the momentum increase resulting in a decrease in the de Broglie wavelength. Given, charge of electron, e = 1.6 xx 10^(-19)C and mass = 9.1 xx 10^(-31) kg . The number of photons emitted per second from a light source of power 40 W and wavelength 5893 Å

Which of the following is correct regarding photons? (i) A photon always travels at a speed 3xx10^(8)m//sec in vacuum. (ii) The rest mass of photon is zero. (iii) Let E and p be the energy and linear momentum of a photon and v and lambda be the frequency and wavelength of the same light when it behaves as a wave. Then E=h v =(hc)/(lambda) p=(h v)/(c) =(h)/(lambda) =(E)/(c) (iv) A photon may collide with a meterial particle. The total energy and the total momentum remain conserved in such a collision. The photon may get absorbed and / or a new photon may be emitted. Thus, the number of photons may not be consverved.