Question

S is isotropic monochromatic source of constant power P. r is very large and v is frequency of radiation .The number of photons crossing per unit time the given surface is in $n=\frac{P\Delta Scos\theta }{k\pi v{r}^{2}h}$ .Find k if $\Delta S$ is the area of small section.

Answer 1

The radiant power at the given surface would be,

$P_1=\frac{P\Delta Scos\theta }{4\pi r^2}$

So number of photons crossing per unit time is, $n=\frac{P_1}{h\nu }=\frac{P\Delta Scos\theta }{4\pi \nu r^{2}h}$

Comparing with the given expression, $k=4$.