A laser produces a 6.00 mW beam of light, consisting of photons with a wavelengt

Question

A laser produces a 6.00 mW beam of light, consisting of photons with a wavelength of 640.8 nm .

Part A

How many photons are emitted by the laser each second?

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Part B

The laser beam strikes a black surface and is absorbed. What is the change in the momentum of each photon that is absorbed?

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Part C

What force does the laser beam exert on the black surface?

A laser produces a 6.00 mW beam of light, consisting of photons with a wavelength of 640.8 nm .

Part A

How many photons are emitted by the laser each second?

n = photons/s

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Part B

The laser beam strikes a black surface and is absorbed. What is the change in the momentum of each photon that is absorbed?

p =   kgm/s  

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Part C

What force does the laser beam exert on the black surface?

Explanation / Answer

A.

E = hc/lambda

Power = N*E

N = no. of photon emitted per sec

N = P/E

N = P*lambda/hc

N = 6*10^-3*640.8*10^-9/(6.625*10^-34*3*10^8)

N = 1.93*10^16 photons/sec

B.

dP = |Pf - Pi|

Pf = -Pi

|dP| = 2*P = 2*h/lambda

dP = 2*6.625*10^-34/(640.8*10^-9)

dP = 2.07*10^-27 kg-m/sec

C.

F = N*dP/dt

F = (1.93*10^16 photons/sec)*(2.07*10^-27 kg-m/sec) = 3.99*10^-11 N

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