Monochromatic light of frequency $6 \times 10^{14} \mathrm{~Hz}$ is produced by a laser. The power emitted is $2 \times 10^{-3} \mathrm{~W}$. How many photons per second on an average, are emitted by the source?

(Given $\mathrm{h}=6.63 \times 10^{-34} \mathrm{Js}$ )

If the energy of the photon is increased by a factor of $4$, then its momentum

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$. Then what inference can you make about their frequencies ? 

In a photoemissive cell with executing wavelength $\lambda $, the fastest electron has speed $v.$ If the exciting wavelength is changed to $\frac{{3\lambda }}{4}$, the speed of the fastest emitted electron will be

According to photon theory of light which of the following physical quantities associated with a photon do not/does not change as it collides with an electron in vacuum