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A. How many photons of a tradiation of w...

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))`?

A

`3 times 10^(19)`

B

`5 times 10^(22)`

C

`2 times 10^(22)`

D

`1.67 times 10^(18)`

Text Solution

Verified by Experts

The correct Answer is:
B
The momentum of photon `=h//lambda`
If n is the number of photons falling per second on the plate, then total momentum per second of the incident photons is
`" "P=n times h/lambda`
Since, the plate is blackened, all photons are absorbed by it.
`" "(DeltaP)/(Deltat)=n""h/lambda`
Since, `F=(DeltaP)/(Deltat)=n""h/lambda`
`therefore n=(Flambda)/h=(6.62 times 10^(-5) times 5 times 10^(-7))/(6.62 times 10^(-34))=5 times 10^(22)`
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