Solve.
-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 ft from a light is 4 foot candles, find the intensity on a screen 20 ft from the light.
Express the answer as a fraction.
A) 4 foot-candles
B) $\frac { 1 } { 5 }$ foot-candle
C) $\frac { 1 } { 4 }$ foot-candle
D) 2 foot-candles

Question

Solve.
-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 ft from a light is 4 foot candles, find the intensity on a screen 20 ft from the light.
Express the answer as a fraction. 
A) 4 foot-candles
B) $\frac { 1 } { 5 }$ foot-candle
C) $\frac { 1 } { 4 }$ foot-candle
D) 2 foot-candles

Quizplus · Accepted Answer

The intensity of light varies inversely as the square of the distance, so $I = \frac{k}{D^2}$, where $k$ is a constant. Given $I = 4$ foot-candles at $D = 5$ ft, we find $k = 4 \times 5^2 = 100$. At $D = 20$ ft, $I = \frac{100}{20^2} = \frac{100}{400} = \frac{1}{4}$ foot-candle.

Solve -The Intensity I of Light Varies Inversely as the Square

Solve
-The Intensity I of Light Varies Inversely as the Square