The rest energy of an electron is

The momentum of a photon of light of frequency $f$ 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?

$A$ radio transmitter transmits at a frequency of $880 \ kHz$ and consumes $10 \ kW$ of power. The number of photons emitted per second from the transmitter is:

If $E$ and $P$ are the energy and the momentum of a photon respectively,then on reducing the wavelength of the photon:

There are two sources of light,each emitting with a power of $100 \, W$. One emits $X$-rays of wavelength $1 \, nm$ and the other visible light at $500 \, nm$. Find the ratio of the number of photons of $X$-rays to the photons of visible light of the given wavelength.