Question

Number of photons of equal energy emitted per second by a 6 mW laser source
operating at wavelength 663 nm is _______.

(Given: h = 6.63 × 10^-34 J·s and c = 3 × 10^8 m/s)

• A. \(10\times10^{15}\)
• B. \(5\times10^{16}\)
• C. \(5\times10^{15}\)
• D. \(2\times10^{16}\)

Hint:

To find photon count, divide laser power by energy of a single photon.

Answer 1

The Correct Option is D

Concept: The energy of a single photon is: \[ E=\frac{hc}{\lambda} \] Power of the source gives the energy emitted per second.
Step 1: Convert given quantities \[ P=6~\text{mW}=6\times10^{-3}~\text{J/s} \] \[ \lambda=663~\text{nm}=663\times10^{-9}~\text{m} \]
Step 2: Energy of one photon \[ E=\frac{(6.63\times10^{-34})(3\times10^8)}{663\times10^{-9}} \approx3.0\times10^{-19}~\text{J} \]
Step 3: Number of photons emitted per second \[ N=\frac{P}{E} =\frac{6\times10^{-3}}{3.0\times10^{-19}} =2\times10^{16} \] Final Answer: \[ \boxed{2\times10^{16}} \]