Question

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

Answer 1

80 cd source should be placed at 80 cm to balance the illuminance.

Explanation:

• The total intensity of the 8 cd and the 12 cd light source is 20 cd.
• If d is the distance of the 80 cd source,
• Therefore, the illuminance (E1) due to the 20 cd source is as follows:

E1 is:

• E1=200.42   …..(1)
• Therefore, the illuminance (E2) due to the 80 cd source is as follows:

E2 is:

• E2=80d2   …..(2)
• As, E1 = E2
• 200.42= 80d2⇒ d = 0.8 m = 80 cm.