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 cm source be placed to balance the illuminance ?

Answer 1

$\text{Totla intensityy of the 8 cd and 12 cd light source=20 cd}$

$\text{Let D=distance of the 80 cd source}$

Now $\frac{20}{\left({0.4}^2\right)}=\frac{80}{d^2}$

$d^2=\frac{80}{20}\times \left({0.4}^2\right)$

$\Rightarrow d=0.8m=80cm$