Question

A parallel beam of monochromatic light of wavelength $500nm$ is incident normally on a perfectly absorbing surface. The power through any cross-section of the beam is $10W$.

The number of photons absorbed per second by the surface is $k\times {10}^{19}$. what is a value of k?

Answer 1

Energy of each photon :-

(a) $E=hc$

$=1242/500$

$=2.48eV$

In $1$ second $10J$ energy passes through any cross - section of the beam,

Thus, the no. of photons $\Rightarrow n=10J/2.48eV$

$n=2.52\times {10}^{19}$

(b) Total moment of all the protons

$p=nh/c=10J/3\times {10}^8$

$p=3.33\times {10}^{-8}Nsec.$