Question

The luminous intensity of a light source is $500Cd$. The illuminance of a surface distant $10m$ from it, will be if light falls normally on it

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

Answer 1

The correct option is A $5$ lux

Illuminance ($E$) is measured in lux. The lux is an SI unit used when characterizing illumination conditions of a surface:
$E=\frac{I}{R^2}lux$
where $I$ is the luminous intensity and $R$ is the distance.
Given $I=500cd$ and $R=10m$
$⟹E=\frac{500}{{10}^2}=5lux$