$A$ particle which has zero rest mass and non-zero energy and momentum must travel with a speed

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?

The antiparticle of an electron is:

Two sources of light emit with a power of $200 \ W$. The ratio of the number of photons of visible light emitted per second by each source having wavelengths $300 \ nm$ and $500 \ nm$ respectively,will be:

When an $X$-ray photon collides with a proton,its frequency:

The momentum of a photon is $2 \times 10^{-16} \text{ g} \cdot \text{cm/s}$. Its energy is: