Question

A lamp is hanging at a height of $4$ $m$ above a table. The lamp is lowered by $1$ $m$. The percentage increase in illuminance is

• A. $40$ %
• B. $64$ %
• C. $78$ %
• D. $92$ %

Answer 1

The correct option is C $78$ %

$Illuminance=illuminatingpower\ast Cos\theta /d^2$

The only change when lamp moves from 4 m to 3 m is its d

Illuminance at 4 m = Power / 16

Illuminance at 3 m = Power / 9

Increase in Illuminance = $\frac{power}{9}-\frac{power}{16}$ = $\frac{7Power}{144}$

So % increase = $\frac{7Power}{144}\ast \frac{16\ast 100}{Power}$ = 77.77 %

Answer. C) 78 %