Question

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?

Answer 1

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 × 12s

= 12 W × t

`rArrt=(10Wxx12s)/(12W)=10s`

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