Question

He-Ne Laser device emits photons of wave length 632.8 nm by rate 4.5 x 1020 photon/s, so
the power of the laser beam =
a. 3.14 W
b. 141.3 W
c. 314.1 w
d. 431.4 W​

Answer 1

Answer: Option b.

Explanation:

We know:

Wavelength = 632.8 nm

Fluence = 4.5*10^20 photon/s

The energy of a single photon of wavelength λ is:

E = (h*c)/λ

where:

h = 6.6*10^(-34) J*s

c = 3*10^8 m/s

And we should rewrite the wave length in meters, so:

λ = 632.8 nm = 632.8*10^(-9) m

replacing these in the energy equation, we get:

E = (6.6*10^(-34)J*s)*(3*10^8 m/s)/(632.8*10^(-9) m) = 3.13*10^(-19) J

So each one of the  4.5x10^20 photon that flow each second have this energy, then the power is:

P = (3.13*10^(-19) J)*(4.5*10^20 /s) = 140.85 J/s

and 1 W = 140.85 J/S

Then the power is:

P = 140.85 W

Then the correct answer is the option b, where the units are a little bit different than mine because I used really simplified values for c and h.