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