Q. An electric bulb illuminates a plane surface. The intensity of illumination on the surface at a point 2 m away from the bulb $5 \times 10^{- 4}\ $phot (lumen$\ cm^{- 2}$). The line joining the bulb to the point makes an angle of $60{^\circ}$ with the normal to the surface. The intensity of the bulb in candela (candle power) is
A) $40 \times 10^{- 4}$
B) 40
C) 40$\sqrt{3\ }\ $
D) 20
Explanation
(b) $5 \times 10^{- 4} = \frac{I\cos{60{^\circ}}}{200 \times 200}$
Or $I = 5 \times 10^{- 4} \times 4 \times 10^{4} \times 2 = 40\ $cd
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.