A helium-neon laser has a power output of 1mW of light of wavelength 632.8 nm.
Q. Calculate the energy of each photon in electron volt.

Explanation

Here, `lamda=632.8nm=632.8xx10^-9m`,
Power, `P=9.42mW=9.42xx10^-3W`
a. Now, energy of each photon in the light beam,
`E=(hc)/(lamda)=(6.62xx10^-34xx3xx10^8)/(632.8xx10^-9)`
`=3.14xx10^-19J=(3.14xx10^-19)/(1.6xx10^-9)=1.963eV`
Also, momentum of each photon in the light beam,
`p=(h)/(lamda)=(6.62xx10^-34)/(632.8xx10^-9)=1.046xx10^-27kgms^-1`
b. The number of photons falling on the target per second,
`N=(P)/(E)=(9.42xx10^-3)/(3.14xx10^-19)=3xx10^16s^-1`
c. The required speed of the hydrogen atom,
`V=(p)/(m_H)=(1.046xx10^-27)/(1.66xx10^-27)=0.63ms^-1`