Question

The intensity of light produced by a light source varies inversely as the square of the distance from the source. If the intensity of light produced 3 feet from a light source is 750 foot-candles, find the intensity of light produced 5 feet from the same source. [4 MARKS]

Answer 1

Concept: 1 Mark
Application: 3 Marks

$\begin{array}{cc}\text{Intensity of light in foot candles}& \text{Distance from the source}\\ 750& 3\\ y& 5\end{array}$

$750\propto \frac{1}{3^2}$

$750=\frac{K}{9}$

$K=750\times 9$

$y\propto \frac{1}{5^2}$

$y=\frac{K}{25}$

$y=\frac{750\times 9}{25}$

y = 270 foot candles