Question

A point source of $100 \,W$ emits light with $5 \%$ efficiency. At a distance of $5\, m$ from the source, the intensity produced by the electric field component is:

• A. $\frac{1}{40 \pi} \frac{W}{m^2}$
• B. $\frac{1}{10 \pi} \frac{W}{m^2}$
• C. $\frac{1}{20 \pi} \frac{W}{m^2}$
• D. $\frac{1}{2 \pi} \frac{W}{m^2}$

Answer 1

Answer: (A)

$I _{ EF }=\frac{1}{2} \times \frac{5}{4 \pi \times 5^2} $
$ =\frac{1}{40 \pi} W / m ^2$