Question

About 5% of the power of a 100 W light bulb is converted to visible radiation. What is the
average intensity of visible radiation (a) at a distance of 1 m from the bulb?
(b) at a distance of 10 m?
Assume that the radiation is emitted isotropically and neglect reflection.

Answer 1

Power rating of bulb, P = 100 W 

It is given that about 5% of its power is converted into visible radiation. 

Power of visible radiation,

Hence, the power of visible radiation is 5W.
(a) Distance of a point from the bulb, d = 1 m 

Hence, intensity of radiation at that point is given as:

(b) Distance of a point from the bulb, d₁ = 10 m
Hence, intensity of radiation at that point is given as: