Question

Two light sources with equal luminous intensity are lying at a distance of $1.2$ $m$ from each other. Where should a screen be placed between them such that the illuminance on one of its faces is four times that on the another face?

• A. $0.2$ $m$
• B. $0.4$ $m$
• C. $0.8$ $m$
• D. $1.6$ $m$

Answer 1

Answer: (B), (C)

The correct options are
B $0.4$ $m$
C $0.8$ $m$

According to the problem
$\frac{I_A}{x^2}=4\frac{I_B}{(1.2-x{)}^2}$
$\Rightarrow \frac{1}{x^2}=\frac{4}{(1.2-x{)}^2}$
$\Rightarrow \frac{1}{x}=\frac{2}{1.2-x}\Rightarrow x=0.4m$ and $1.2-x=0.8m$