JEE Main & Advanced
Physics
Ray Optics
Question Bank
Photometry
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 \[1.2-x=0.8\,m\].