Question

1% of light of a source with luminous intensity 50 candela is incident on a circular surface of radius 10 cm. The average illuminance of surface is

• A. 100 lux
• B. 200 lux
• C. 300 lux
• D. 400 lux

Answer 1

Answer: (B)

200 lux

Answer 2

$\varphi = 4\pi L = 200\mspace{6mu}\pi$lumen.

so $I = \frac{\varphi}{100A} = \frac{200\pi}{100 \times \pi r^{2}} = \frac{2}{(0.1)^{2}}$= 200 lux.