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A totally reflecting, small plane mirror...

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. `

Text Solution

Verified by Experts

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`
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