question_answer

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 illuminance on one of its faces is four times that on another face 

A)            0.2 m                                       

B)            0.4 m

C)            0.8 m                                       

D)            1.6 m

Correct Answer: C

Solution :

                   According to the problem                    \[\frac{{{I}_{A}}}{{{x}^{2}}}=4\frac{{{I}_{B}}}{{{(1.2-x)}^{2}}}\]                    \[\Rightarrow \frac{1}{{{x}^{2}}}=\frac{4}{{{(1.2-x)}^{2}}}\]            \[\Rightarrow \frac{1}{x}=\frac{2}{1.2-x}\]\[\Rightarrow x=0.4\,m\] and \[1.2-x=0.8\,m\].