A $2 \mathrm{~mW}$ laser operates at a wavelength of $500 \mathrm{~nm}$. The number of photons that will be emitted per second is :
[Given Planck's constant $\mathrm{h}=6.6 \times 10^{-34} \mathrm{Js}$, speed of light $c=3.0 \times 10^{8} \mathrm{~m} / \mathrm{s}$ ]
Correct Option: 1,
(1) Energy of photon (E) is given by
$E=\frac{h c}{\lambda}$
Number of photons of wavelength $\lambda$ emitted in $t$ second from laser of power $P$ is given by
$n=\frac{P t \lambda}{h c}$
$\Rightarrow n=\frac{2 \times \lambda}{h c}=\frac{2 \times 10^{-3} \times 5 \times 10^{-7}}{2 \times 10^{-25}}(\because t=1 \mathrm{~S})$
$\Rightarrow n=5 \times 10^{15}$
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