A totally reflecting, small plane mirror placed horizontally faces a parallel beam of light as shown in the Fig. The mass of the mirror is 20 g. Assume that there is no absorption in in the lens and that `30%` of the light emitted by the source goes through the lens. Find the power (in`xx10^(8)`W) of the source needed to support the weight of the mirror. Take `g=10ms^(-2)`

Explanation

Let n photons (each of frequency f) per second are emitted from source. Then power of source is `P=nhf`
But on `30%` of the photons go towards mirror. The force exerted on mirror is
`F=2[(30)/(100)n](h)/(lamda)=(3)/(5)(nhf)/(c)=(3)/(5)(P)/(c)`
and this force should be equal weight of mirror, so
`(3)/(5)(P)/(c)=20xx10^(-3)g`
`impliesP=(5xx3xx10^(8)xx20xx10^(-3)xx10)/(3)=10^(8)W`