12.(b) About 5% of the power of a 100 W light bulb is converted to visible
radiation. What is the average intensity of visible radiation at a distance of 10 m?
Assume that the radiation is emitted isotropically and neglect
reflection.

Answers (1)

Total power which is converted into visible radiation = 5% of 100W = 5W

The above means 5J of energy is passing through the surface of a concentric sphere(with the bulb at its centre) per second.

Intensity for a sphere of radius 10 m

 =\frac{5}{4\pi (10)^{2}}

=3.98\times10-3 Wm-2

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