Question

The illuminance on screen distance $3$ m from a $100$ $W$ lamp is $25$ lm/$m^2$. Presuming normal incidence, the luminous intensity of the bulb will be

• A. $100$ $Cd$
• B. $25$ $Cd$
• C. $225$ $Cd$
• D. none of these

Answer 1

Answer: (C)

$225$ $Cd$

We have illuminance
$I=\frac{\varphi }{R^2}$
$⟹\varphi =I{R}^2=25\times 3^2=225Cd$