An electric lamp and a candle produce equal illuminance at a photometer screen when they are placed at 80 cm and 20 cm from the screen respectively. The lamp is now covered with a thin paper which transmits 49% of the luminous flux. By what distance should be lamp be moved to balance the intensities at the screen again?

Question

An electric lamp and a candle produce equal illuminance at a photometer screen when they are placed at 80 cm and 20 cm from the screen respectively. The lamp is now covered with a thin paper which transmits 49% of the luminous flux. By what distance should be lamp be moved to balance the intensities at the screen again?

Philoid · Accepted Answer

Let be the intensity when placed at a distance 80 cm and be the intensity when placed at a distance 20 cm apart from the screen.

Now,

According to the question, let the new distance between the lamp and the screen be x such that even after covering the lamp with a thin paper the intensities at the screen is balanced.

So,

⇒

Thus, the lamp has to be moved by 80 cm – 56 cm = 24 cm.