Q. Correct exposure for a photographic print is $10\ seconds$ at a distance of one metre from a point source of $20\ candela$. For an equal fogging of the print placed at a distance of $2\ m$ from a $16\ candela$ source, the necessary time for exposure is
A) $100\ s$
B) $25\ s$
C) $50\ s$
D) $75\ s$
Explanation
(c) For equal fogging $I_{2} \times t_{2} = I_{2} \times t_{2}$
$\Rightarrow \frac{L_{2}}{r_{2}^{2}} \times t_{2} = \frac{L_{1}}{r_{1}^{2}} \times t_{1} \Rightarrow \frac{16}{4} \times t_{2} = \frac{20}{1} \times 10$
$\Rightarrow t_{2} = 50s$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.