Optics Question 479
Question: 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
Options:
A) 100 sec
B) 25 sec
C) 50 sec
D) 75 sec
Show Answer
Answer:
Correct Answer: C
Solution:
For equal fogging $ I _{2}\times t _{2}=I _{1}\times t _{1} $
$ \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}=50\sec . $
