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
⇒ 
Therefore, the photographic plate should be exposed for 10 s to get equally intense lines.
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