Question

Light from a lamp is falling normally on a surface distant $10$ m from the lamp and the luminous intensity on it is $10$lux. In order to increase the intensity $9$ times, the surface will have to be placed at a distance of

• A. $10$ $m$
• B. $\frac{10}{3}$ $m$
• C. $\frac{10}{9}$ $m$
• D. $10\times 9$ $m$

Answer 1

The correct option is B $\frac{10}{3}$ $m$

$I=\frac{LCos\theta }{r^{\circ }}$

Since light falls normally $Cos\theta =1$

$10=\frac{L}{100}$ => L = 1000Cd

$90=\frac{L}{d^2}$

$⟶d^2=1000/90$

$d=\frac{10}{3}$

Answer B) $\frac{10}{3}m$