Question

A lamp of luminous intensity $20$ $Cd$ is hanging at a height of $40$ $cm$ from the center of a square table of side $60$ $cm$. The illuminance at the centre of the table will be

• A. $100$ lux
• B. $125$ lux
• C. $150$ lux
• D. none of these

Answer 1

Answer: (B)

$125$ lux

Luminous intensity, L = 20 Cd

Distance of source from center, d = 40 cm

Light from lamp hits the center normal ($\theta =0^{\circ }$)

Lambert's law, $I=LCos\theta /d^2$

$⟶I=20Cos{0}^{\circ }/(0.4{)}^2$

$⟶I=125lux$

Answer. B) 125 lux