Question

The light intensity of a source is 100 candelas. The illuminance on a surface is 4 lux. How far is the surface from the source?

A. 7 meters
B. 6 meters
C. 4 meters
D. 5 meters

Answer 1

The correct answer is D. 5 meters.

Answer 2

Answer : Option D) 5 meters

Explanation :

To find the luminous intensity this formula is used;

Luminous intensity = illuminance ×
`radius ^(2)`

∴ Lv = Ev ×
`r^(2)`

We know the luminous intensity is 100 candelas and the illuminance is 4 lux.

∴ 100 = 4 x
`r^(2)`

∴ 100/4 =
`r^(2)`

∴ 25 =
`r^(2)`

∴ r =√25 = 5 meters

Therefore, the surface will be at a distance of 5 meters from the source.