The proper exposure time for a photographic print is 20 s at a distance of 0.6 m from a 40 candle power lamp. How long will you expose the same print at a distance of 1.2 m from a 20 candle power lamp ?

Explanation

In case of camera for proper exposure
`I_(1) D_(1) .^(2)t_(1) = I_(2) D_(2) .^(2)t_(2)`
As here D is constant and I = `(L//r^(2))`
`(L_(1))/(r_(1)^(2)) xx t_(1) = (L_(2))/(r_(2)^(2)) xx t_(2)`
So `(40)/((0.6)^(2)) xx 20 = (20)/((1.2)^(2)) t , i.e., t= 160 s`