A point source causes photoelectric effect

 

Question :- A point source causes photoelectric effect from a small metal plate. Which of the following curves may represent the saturation photocurrent as a function of the distance between the source and the metal?

Solution :- If number of photons falling on the plate increases then the number of photo electrons increases, increasing the photo electric current.

As intensity is given by :

From above equation,

Intensity (I) ∝ number of photons incident(n)     …..(1)

Also photoelectric current

i ∝ number of electrons ejected ∝ number of photons incident(n)     …..(2)

So from (1) and (2), we get

i ∝ I ∝ 1/r²

Hence curve d represent the photocurrent as a function of the distance between the source and the metal.