Question

A small plate of a metal is placed at a distance of 2m from a monochromatic light source of wavelenght `4.8xx10^(-7)m` and power 1.0 Watt. The light falls normally on the plate. Find the number of photons striking the metal plate per square metre per second.

Answer 1

Correct Answer - A::B::D
Number of photons emitted per second,
`N_1 = (Power of source )/(Energy of one photon) = (P)/((hc/lambda)) = (Plambda)/(hc)`
At a distance r, these photons are falling on an area
`4pir^2`.
`:.` Number of photons incident per unit area per
unit time, `N_2 = (N_1)/(4pir^2) = (P lambda)/((4pir^2))hc