Question

12) The illumination, I, produced by a light source varies inversely as the square of the distance, D from the light source. Suppose the illumination, I, was 88 footcandles when the distance, D, from the light source was 5 feet. Find the illumination, I, produced when the distance, D, is 4 feet from the same source. Round answer to the nearest tenth of a footcandle.

Answer 1

Solution:

$\small I\propto \frac{1}{D}$

$\small \therefore I= \frac{k}{D}$ , where  $\small k$ is the constant of proportionality.

when   

1=88, D=5

5 8

k = 88 x 5 = 440

To find  $\small I$ when

D=4

$\small I=\frac{k}{D}$

440

I- 110 footcandles