$A$ photon in motion has a mass.

The momentum of a photon of energy $1 \, MeV$ in $kg \, m/s$ will be:

The energy of a photon with a wavelength of $450 \, nm$ is ............

Estimating the following two numbers should be interesting. The first number will tell you why radio engineers do not need to worry much about photons! The second number tells you why our eye can never 'count photons',even in barely detectable light.
$(a)$ The number of photons emitted per second by a Medium wave transmitter of $10\; kW$ power,emitting radiowaves of wavelength $500\; m$.
$(b)$ The number of photons entering the pupil of our eye per second corresponding to the minimum intensity of white light that we humans can perceive $(10^{-10}\; W m^{-2})$. Take the area of the pupil to be about $0.4\; cm^2$,and the average frequency of white light to be about $6 \times 10^{14}\; Hz$.

An important spectral emission line has a wavelength of $21 \ cm$. The corresponding photon energy is $(h = 6.62 \times 10^{-34} \ J \cdot s; \ c = 3 \times 10^8 \ m/s)$.

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