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.

Explanation

Here, power of radiation = 5% of electrical power
`=100xx(5)/(100)`
`therefore P=5W`
Again, according to formula,
`I=(P)/(4pi r^(2))`
`=(5)/(4xx3.14xx(10)^(2))(because r =10 m)`
`therefore I=3.981xx10^(-3)Wm^(-2)~~0.004 Wm^(-2)`