The velocity of photons is proportional to (where $\nu=$ frequency)

Assertion: Mass of a moving photon varies inversely as the wavelength.
Reason: Energy of the particle $= \text{mass} \times (\text{speed of light})^2$

In pair annihilation,an electron and a positron destroy each other to produce gamma radiation. How is the momentum conserved?

The momentum of a photon having energy $hv$ is ............

The energy of a photon of light of wavelength $450 \ nm$ is

$A$ $160 \,W$ infrared source is radiating light of wavelength $50000 \,\mathring A$ uniformly in all directions. The photon flux at a distance of $1.8 \,m$ is of the order of ............. $m^{-2} s^{-1}$.