A satisfactory photographic print is obtained when the exposure time is `10 sec` at a distance of `2m` from a `60 cd` lamp. The time of exposure required for the same quality print at a distance of `4m` from a `120 cd` lamp isA. `5 sec`B. `10 sec`C. `15 sec`D. `20sec`

A satisfactory photographic print is obtained when the exposure time i

1.	A satisfactory photographic print is obtained when the exposure time is `10 sec` at a distance of `2m` from a `60 cd` lamp. The time of exposure required for the same quality print at a distance of `4m` from a `120 cd` lamp isA. `5 sec`B. `10 sec`C. `15 sec`D. `20sec`
Answer» Correct Answer - D `I_(1)D_(1)^(2)t_(1)=I_(2)D_(2)^(2)t_(2)` Here `D` is constant and `I=(L)/(r^(2))` So `(L_(1))/(r_(1)^(2))xxt_(1)=(L_(2))/(r_(2)^(2))xxt_(2)` `implies(60)/((2)^(2))xx10=(120)/((4)^(2))xxt=20sec`

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A satisfactory photographic print is obtained when the exposure time is `10 sec` at a distance of `2m` from a `60 cd` lamp. The time of exposure required for the same quality print at a distance of `4m` from a `120 cd` lamp isA. `5 sec`B. `10 sec`C. `15 sec`D. `20sec`

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