The intensity I of light varies inversely as the square of the distance $D$ from the source. If the intensity of illumination on a screen 5 feet from a light is 4 foot candles, find the intensity on a screen 20 feet from the light.
A) $1 \frac{1}{4}$ foot- candles
B) 2 foot- candles
C) $\frac{1}{5}$ foot- candles
D) $\frac{1}{4}$ foot- candles

Question

Quizplus · Accepted Answer

The intensity of light varies inversely as the square of the distance, so $I_1D_1^2 = I_2D_2^2$. Given $I_1 = 4$ foot-candles at $D_1 = 5$ feet, and $D_2 = 20$ feet, solving for $I_2$ gives $I_2 = \frac{4 \times 5^2}{20^2} = \frac{1}{4}$ foot-candles.

The Intensity I of Light Varies Inversely as the Square DDD

The Intensity I of Light Varies Inversely as the Square $D$