Q. A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with this distance $r$ from the source as
A) $\propto \frac{1}{r}$
B) $\propto \frac{1}{r^{2}}$
C) $\propto \frac{1}{r^{3}}$
D) $\propto \frac{1}{r^{4}}$
Explanation
(c) $E = \frac{I\cos\theta}{r^{2}} \Rightarrow E = \frac{Ih}{r^{3}}$
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Or $E \propto \ \frac{1}{r^{3}}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.