Question
A photographic plate records sufficiently intense lines when it is exposed for 12s to a source of 10W. How long should it be exposed to a 12W source radiating the light of same colour to get equally intense lines?
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Answer
To get equally intense lines on the photographic plate, the radiant flux (energy) should be same. So, 10W × 12sec = 12W × t$\Rightarrow \text{t}=\frac{10\text{W}\times12\text{sec}}{12\text{W}}=10\text{sec.}$
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