When a photon enters glass from air,which one of the following quantities does not change?

$A$ photon of frequency $f$ has a momentum associated with it. If $c$ is the velocity of light,the momentum is:

An electron-positron pair is created by $\gamma$-rays. What is the minimum energy of the $\gamma$-rays required for this process in $MeV$?

Which one of these is non-divisible?

Monochromatic light of frequency $6.0 \times 10^{14} \;Hz$ is produced by a laser. The power emitted is $2.0 \times 10^{-3} \;W$.
$(a)$ What is the energy of a photon in the light beam?
$(b)$ How many photons per second,on an average,are emitted by the source?

$A$ photon of wavelength $\lambda$ is scattered from an electron,which was at rest. The wavelength shift $\Delta \lambda$ is three times of $\lambda$ and the angle of scattering $\theta$ is $60^o$. The angle at which the electron recoiled is $\phi$. The value of $\tan \phi$ is: (electron speed is much smaller than the speed of light)