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 5×104 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) 403

B) 40

C) 20

D) 40×104

Show Answer

Answer:

Correct Answer: B

Solution:

I=Lcosθr2

  L=I×r2cosθ

=5×104×104×22cos60 = 40 Candela



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