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 . $



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