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.

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