Question

A sodium lamp emits $3.14\times {10}^{20}$ photons per second. Calculate the distance from the sodium lamp where flux of photon is one photon per second per $c{m}^2$

• A. $5cm$
• B. $5\times {10}^{9}cm$
• C. $10cm$
• D. ${10}^{7}cm$

Answer 1

The correct option is B $5\times {10}^{9}cm$

Photon flux at a distance r
$n=\frac{no.ofphotonsemittedpersec}{4\pi r^2}\Rightarrow 1=\frac{3.14\times {10}^{20}}{4\times 3.1\times r^2}\Rightarrow r^2=\frac{{10}^{20}}{4}\Rightarrow r=5\times {10}^{9}cm$