Question

How many photons are emitted per second by a 3 mW laser source operating at 663 nm?

1. 1 × 10¹⁶
2. 3.6 × 10¹⁶
3. 2 × 10¹⁶
4. 5.2 × 10¹⁶

Answer 1

Correct Answer - Option 1 : 1 × 10¹⁶

The energy of each photon is

\(\begin{array}{l} E\; = \;\frac{{hc}}{\lambda }\\ \; = \;\frac{{6.63 \times {{10}^{ - 34}} \times 3 \times {{10}^8}}}{{663 \times {{10}^{ - 9}}}}\; = \;3 \times {10^{ - 19}}J \end{array}\) 

The energy of the laser emitted per second is 3 × 10 ^{- 3}J

Thus the number of photons emitted per second

\(= \;\frac{{3 \times {{10}^{ - 3}}}}{{3 \times {{10}^{ - 19}}}}\; = \;{10^{16}}\)