A photographic plate records sufficiently intense lines when it is exposed for 12 s to a source of 10 W. How long should it be exposed to a 12 W source radiating the light of same colour to get equally intense lines?

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Solution

Let t be the time for which the photograph is exposed.
To record the sufficiently intense lines, energy should be same.
Energy = radiant flux × time
= 10 W×12 s
= 12 W×t
$\Rightarrow t = \frac{10 W \times 12 s}{12 W} = 10 s$
Therefore, the photographic plate should be exposed for 10 s to get equally intense lines.

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A photographic plate records sufficiently intense lines when it is exposed for 12 s to a source of 10 W. How long should it be exposed to a 12 W source radiating the light of same colour to get equally intense lines?

Let t be the time for which the photograph is exposed. To record the sufficiently intense lines, energy should be same. Energy = radiant flux × time = 10 W×12 s = 12 W×t ⇒ t=10 W×12 s12 W =10 s Therefore, the photographic plate should be exposed for 10 s to get equally intense lines.