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 2 foot-candles, find the intensity on a screen 15 ft from the light.
A) $\frac { 2 } { 9 }$ foot-candle
B) $\frac { 2 } { 10 }$ foot-candle
C) $1 \frac { 2 } { 9 }$ foot-candles
D) 2 foot-candles

Question

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 2 foot-candles, find the intensity on a screen 15 ft from the light. 
A) $\frac { 2 } { 9 }$ foot-candle
B) $\frac { 2 } { 10 }$ foot-candle
C) $1 \frac { 2 } { 9 }$ foot-candles
D) 2 foot-candles

Quizplus · Accepted Answer

The intensity of light varies inversely as the square of the distance, so if the distance triples (from 5 ft to 15 ft), the intensity decreases by a factor of $3^2 = 9$. Therefore, the new intensity is $\frac{2}{9}$ foot-candle.

The Intensity I of Light Varies Inversely as the Square 29\frac { 2 } { 9 }92​

The Intensity I of Light Varies Inversely as the Square $\frac { 2 } { 9 }$