question_answer

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]

A)            \[40\sqrt{3}\]                      

B)            40

C)            20  

D)            \[40\times {{10}^{-4}}\]

Correct Answer: B

Solution :

                   \[I=\frac{L\cos \theta }{{{r}^{2}}}\] \[\Rightarrow \ \ L=\frac{I\times {{r}^{2}}}{\cos \theta }\] \[=\frac{5\times {{10}^{-4}}\times {{10}^{4}}\times {{2}^{2}}}{\cos 60{}^\circ }\ =\ 40\ Candela\]