Optics Question 513

Question: A lamp is hanging at a height of 4m above a table. The lamp is lowered by 1m. The percentage increase in illuminace will be

Options:

A) 40 %

B) 64%

C) 78%

D) 92%

Show Answer

Answer:

Correct Answer: C

Solution:

$ I _{1}=\frac{L}{r _{1}^{2}}=\frac{L}{16} $

and $ I _{2}=\frac{L}{r _{2}^{2}}=\frac{L}{9} $

% increase in illuminance $ =\frac{I _{2}-I _{1}}{I}\times 100 $

$ =( \frac{16}{9}-1 )\times 100 $

$ \approx 78% $



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