Optics Question 477
Question: An electric bulb illuminates a plane surface. The intensity of illumination on the surface at a point 2m away from the bulb is $ 5\times {{10}^{-4}} $ phot (lumen/cm2). The line joining the bulb to the point makes an angle of 60o with the normal to the surface. The intensity of the bulb in candela is
[IIT-JEE 1980; CPMT 1991]
Options:
A) $ 40\sqrt{3} $
B) 40
C) 20
D) $ 40\times {{10}^{-4}} $
Show Answer
Answer:
Correct Answer: B
Solution:
$ I=\frac{L\cos \theta }{r^{2}} $
$ \Rightarrow \ \ L=\frac{I\times r^{2}}{\cos \theta } $
$ =\frac{5\times {{10}^{-4}}\times 10^{4}\times 2^{2}}{\cos 60{}^\circ }\ =\ 40\ Candela $
