Question

Two lamps of luminous intensity of 8 Cd and 32 Cd respectively are lying at a distance of 1.2 m from each other. Where should a screen be placed between two lamps such that its two faces are equally illuminated due to two sources

• A. 10 cm from 80 Cd lamp
• B. 10 cm from 32Cd lamp
• C. 40 cm from 8 Cd lamp
• D. 40 cm from 32 Cd lamp

Answer 1

Answer: (C)

40 cm from 8 Cd lamp

Answer 2

$I = \frac{L}{r^{2}} \Rightarrow \frac{L_{1}}{L_{2}} = \frac{r_{1}^{2}}{r_{2}^{2}}$ or $\frac{8}{x^{2}} = \frac{32}{(120 - x)^{2}}$

Solving it we get $x = 40cm$.