A focused 100 W spotlight delivers 50% of its light into a circle with a diameter of 50 cm.What is the average rms electric field in this illuminated area?

Question

Quizplus · Accepted Answer

The average intensity is given by:$$I=\frac{P}{A}=\frac{50	ext{ W}}{\pi(0.25	ext{ m})^2}=637	ext{ W/m}^2$$The rms electric field is given by:$$E_{rms}=\sqrt{\frac{2I}{\epsilon_0 c}}=\sqrt{\frac{2(637	ext{ W/m}^2)}{8.85	imes10^{-12}	ext{ F/m}(3	imes10^8	ext{ m/s})}}=190	ext{ V/m}$$

A Focused 100 W Spotlight Delivers 50% of Its Light