Question

A photocell is receiving light from a source placed at a distance of $1m$ . If the same source is to be placed at a distance of $2m$ , then the ejected electrons

• A. moves with one-fourth energy as that of the initial energy.
• B. moves with one-fourth of momentum as that of the initial momentum.
• C. will be half in number.
• D. will be one-fourth in number.

Answer 1

Answer: (D)

$\text{Number of ejected electrons}\propto \left(\text{Intensity}\right)\propto \frac{1}{{\left(\text{Distance}\right)}^2}$
Therefore an increment of distance two times will reduce the number of ejected electrons to $1/4^{th}$ of the previous one.