Question

A point source emitting uniformly in all directions is placed 0.5 m above a table top. The luminous flux of the source is $500$ $\pi$ lumen. Find the illumnance at a small surface area of the table top 0.8 m from the source

• A. $125lux$
• B. $122lux$
• C. $118lux$
• D. $115lux$

Answer 1

Answer: (B)

$122lux$

$I=\frac{500\pi }{4\pi }=125cd$
$E=\frac{Icos\theta }{r^2}=\frac{125\times 5}{(0.8{)}^2\times 8}$
$=122lux$