Q. An movie projector forms an image $3.5m$ long of an object $35mm$. Supposing there is negligible absorption of light by aperture then illuminance on slide and screen will be in the ratio of
A) $100\ :1$
B) $10^{4}\ :1$
C) $1\ :100$
D) $1\ :10^{4}$
Explanation
(b) $I \propto \frac{1}{r^{2}}$ so,
$\frac{Illuminance\ on\ slide}{Illuminance\ on\ screen} = \frac{(Length\ of\ image\ on\ screen)^{2}}{(Length\ of\ object\ on\ slide)^{2}}$
$= \left( \frac{3.5\ m}{35\ mm} \right)^{2} = 10^{4}:1$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.