Answer
(a) $2.39\times 10^{16}$photons per second.
(b) $2.10\times 10^{-27}Kgm/s$
(c) $5.0\times 10^{-11}N$
Work Step by Step
(a) We know that
$Energy=\frac{hc}{\lambda}$
$Energy=\frac{6.63\times 10^{-34}\times 3\times 10^8}{632.8\times 10^{-9}}=3.14\times 10^{-19}J$
Now, $Power=n\times Energy$
$\implies 7.5\times 10^{-3}=n\times 3.14\times 10^{-19}$
This simplifies to:
$n=2.39\times 10^{16}$photons per second.
(b) We know that
$p=\frac{h}{\lambda}$
$\implies p=\frac{6.63\times 10^{-34}}{632.8\times 10^{-9}}$
$p=1.05\times 10^{-27}Kgm/s$.
Since the photon strikes and reflects back from the mirror, the change in momentum is twice -- that is, $2.10\times 10^{-27}Kgm/s$
(c) We can find the required force as
$F=n\times p$
We plug in the known values to obtain:
$F=2.39\times 10^{16}\times 2.10\times 10^{-27}=5.0\times 10^{-11}N$
