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

Explanation

Here `m = 20 g = 20 xx 10^(-3) kg `
The weight of the mirror is balanced. So, force exerted by the photons is equal to weight .
` P = h / lambda and E=(hc)/(lambda) = Pc`
`implies E/t = (P)/(t)c`
`implies `Rate of change of momentum
= power//c`
`30%` of light passes through the lens.
Thus it exerts force, `70%` is reflected
`:. Force exerted - 2
(rate of change of momentum)
`= 2xxPower//c`
`30%((2xxPower)/(theta)) =mg`
`implies Power = (20 xx 10^(-3)xx10xx3xx10^(3)xx10)/(2 xx 3)`
`=10 W power = 100 MW`.