Question

A point source emitting light uniformly in directions is placed 60 cm above-top. The illuminance at a point on the table-top , directly below the source, is 15 lux .Find the illuminance at a point on the table-top 80 cm away from the first point.

Answer 1

Given that :

$EA=15lux=\frac{I_0}{(60{)}^2}$

$\Rightarrow I_0=15\times (0.6{)}^2$

=5.4 candela

So, $EB=10cos\theta /O{B}^2$

$=5.4\left(\frac{3}{5}\right)1^2=3.24lux$