$n$ photons of wavelength $\lambda$ are absorbed by a black body of mass $m$. The momentum gained by the body is

The energy of a photon with a wavelength of $\lambda = 4000 \ \mathring{A}$ is equal to ... $eV$.

$A$ photon in motion has a mass.

Consider a hypothetical annihilation of a stationary electron with a stationary positron. What is the wavelength of the resulting radiation? ($h =$ Planck's constant,$c =$ speed of light,$m_0 =$ rest mass)

The antiparticle of an electron is:

Monochromatic light of wavelength $632.8 \; nm$ is produced by a helium-neon laser. The power emitted is $9.42 \; mW$.
$(a)$ Find the energy and momentum of each photon in the light beam.
$(b)$ How many photons per second,on the average,arrive at a target irradiated by this beam? (Assume the beam to have uniform cross-section which is < target area).
$(c)$ How fast does a hydrogen atom have to travel in order to have the same momentum as that of the photon?