Two light sources of intensities 8 Cd and 12 Cd placed on the same side of a photometer screen at a distance of 40 cm from it. Where should an 80 Cd source be placed to balance the illuminance?

Question

Philoid · Accepted Answer

Total intensity of the 8 Cd and the 12 Cd light source is (8+12) =20 Cd.

Let be the distance of the 80 Cd source.

∴ Illuminance due to the 20 Cd source is:

------------- (1)

∴ Illuminance due to the 80 Cd source is:

--------------- (2)
As