A. How many photons of a tradiation of wavelength `lamda=5xx10^(-7)` m must fall per second on a blackened plate in order to produce a force of `6.62xx10^(-5)`N?
B. At what rate will the temperature of plate rise if its mass is 19.86kg and specific heat is equal to `2500J(kgK^(-1))`?

Options

A. `3xx10^(19)`
B. `5xx10^(22)`
C. `2xx10^(22)`
D. `1.67xx10^(18)`

Answer

B

Explanation

The momentum of photon `=(h)/(lamda)`
If n is the number of photons falling per second on the plate, then total momentum per second of the incident photons is
`P=nxx(h)/(lamda)`
Since the plate is blackened, all photons are absorbed by it.
`(triangleP)/(trianglet)=n(h)/(lamda)`
Since `F=(triangleP)/(trianglet)=n(h)/(lamda)`
`n=(Flamda)/(h)`
`=(6.62xx10^(-5)xx5xx10^(-7))/(6.62xx10^(-34))=5xx10^(22)`