A photographic plate placed at a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is

Question

Philoid · Accepted Answer

Given:

Distance,

Calculation:

Consider the incident illuminance is E₀, illuminance at distance 3 cm and 10 cm are E₁ and E₂ respectively.

Illuminance at 3 cm is given by,

Time is given by,

Thus, time required is,

So, option B is correct.