$A$ $2\,mW$ laser operates at a wavelength of $500\,nm.$ The number of photons that will be emitted per second is [Given Planck's constant $h = 6.6 \times 10^{-34}\,Js,$ speed of light $c = 3.0 \times 10^8\,m/s$]

The frequency of a photon having energy $100 \ eV$ is $(h = 6.6 \times 10^{-34} \ J \cdot s)$.

Monochromatic light of frequency $6 \times 10^{14} \text{ Hz}$ is produced by a laser. The power emitted is $4 \times 10^{-3} \text{ W}$. How many photons per second on an average are emitted by the source? $[h = 6.63 \times 10^{-34} \text{ Js}]$

When light of wavelength $\lambda$ is incident on a photosensitive surface,photons of power $P$ are emitted. The number of photons $n$ emitted in time $t$ is: [$h$ = Planck's constant,$c$ = velocity of light in vacuum]

Two sources of light emit $X$-rays of wavelength $1 \ nm$ and visible light of wavelength $500 \ nm$,respectively. Both sources emit light of the same power $200 \ W$. The ratio of the number of photons emitted per second by the $X$-ray source to the number of photons emitted per second by the visible light source is:

$n$ photons of wavelength $\lambda$ are absorbed by a black body of mass $m$. The momentum gained by the body is