Question

How many photons are emitted per second by a 5 mW laser source operating at 632.8 nm?

• A. $1.6\times {10}^{16}$
• B. $1.6\times {10}^{13}$
• C. $1.6\times {10}^{10}$
• D. $1.6\times {10}^3$

Answer 1

The correct option is A $1.6\times {10}^{16}$

The energy of a photon is $E=\frac{hc}{\lambda }=\frac{(6.62\times {10}^{-34})(3\times {10}^8)}{632.8\times {10}^{-9}}=3.14\times {10}^{-19}J$

Given, the energy emitted per second by laser $=5mW=5\times {10}^{-3}J$

Thus the number of photons emitted per second $=\frac{5\times {10}^{-3}}{3.14\times {10}^{-19}}=1.6\times {10}^{16}$