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 the two sources?

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

Answer 1

The correct option is C $40$ $cm$ from $8$ $Cd$ lamp

Let distance between 8 Cd lamp and screen be $xm$

Distance between 32 Cd lamp and screen $=$ $(120-x)m$

Under equal illumination,

$\frac{8}{x^2}=\frac{32}{(120-x{)}^2}$

=> x $=$ 40 cm

So the distance from 8 Cd lamp is 40 centimeters.