12.(a) 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 1m from the bulb?
Assume that the radiation is emitted isotropically and neglect reflection.
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 1m