Question

A movie projector forms an image 3.5m long of an object 35 mm. 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. 10⁴ : 1
• C. 1 : 100
• D. 1 : 10⁴

Answer 1

Answer: (B)

10⁴ : 1

Answer 2

$I \propto \frac{1}{r^{2}}$

So, $\frac{\text{Illuminance on slide }}{\text{Illuminance on screen}} = \frac{(\text{Length of image on screen})^{2}}{(\text{Length of object on slide})^{2}} = \left( \frac{3.5\mspace{6mu} m}{35\mspace{6mu} mm} \right)^{2} = 10^{4}:1$