Solve the problem.
-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 $5 \mathrm { ft }$ from a light is 4 foot candles, find the intensity on a screen 20 ft from the light.
A) $\frac { 1 } { 5 }$ foot-candle
B) $1 \frac { 1 } { 4 }$ foot-candles
C) $\frac { 1 } { 4 }$ foot-candle
D) 2 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 $D_1 = 5$ ft, and $D_2 = 20$ ft, solve for $I_2$: $4 \times 5^2 = I_2 \times 20^2$, resulting in $I_2 = \frac{1}{4}$ foot-candle.

Solve the Problem 5ft5 \mathrm { ft }5ft From a Light Is 4 Foot Candles, Find the Intensity

Solve the Problem $5 \mathrm { ft }$ From a Light Is 4 Foot Candles, Find the Intensity