A photographic plate is placed directly in from of a small diffused source in the shape of a circular disc. It takes 12 s to get a good exposure. If the source is rotated by 60° about one of its diameters, the time needed to get the same exposure will be
Given:
Time, t1 = 12 s
Angle, θ1=00
Angle, θ2=600
Calculation:
Consider the incident luminosity as E0 at distance r.
The luminosity is given as,
The time is given by,
And
t2=12×2=24 s
So, option C is correct.