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 esmitted isotropically and neglect reflection.

Explanation

Effective power(energy radiated per second)
`5%` of `100 W`
`P= 5 W`
this energy will distribute on a sphere. At a distance `r` From the point source, area on which light is incident is
`S=4 pir^2`
Intensity at diastance `r` from the point source,
`:. I= P/S=5/(4pir^2)` =Energy incident per unit area per unit time
(a) At `r = 1 m`,
`I = 5/(4pi(1)^2`
`= 0.4 W//m^2`
At `r = 10 m`,
`I = 5/(4pi(10)^2`
`0.004 W//m^2`
.