Question

A battery-operated torch is adjusted to send an almost parallel beam of light. It produce an illuminance of $40lux$ when light falls on a wall $2m$ away. The illuminance produced when it falls on a wall $4m$ away is close to

• A. $40lux$
• B. $20lux$
• C. $10lux$
• D. $5lux$

Answer 1

The correct option is A $40lux$

$Illuminance=\frac{LuminousPower}{Area}$

Since the beam of light is parallel, so the area does not change with distance. Also the power of torch being same, the luminous power remains same.

Therefore illuminance remains same on a wall 2 m away and 4 m away.

Illuminance produced on a wall 4 m away = 40 lux

Answer. A) 40 lux