What is a photon?

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

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?

Two monochromatic beams $A$ and $B$ of equal intensity $I$ hit a screen. The number of photons hitting the screen by beam $A$ is twice that by beam $B$. What inference can you make about their frequencies?

$A$ photon falls through a height of $1 \, km$ in the earth's gravitational field. To calculate the change in its frequency,take its mass to be $h \nu / c^{2}$. The fractional change in frequency $\nu$ is close to

The ratio of energies of photons with wavelengths $\lambda_1 = 150 \ nm$ and $\lambda_2 = 300 \ nm$ is: