Question

A laser used to weld detached retinas emits light with a wavelength of $652nm$ in pulses that are $20.0ms$ in duration. The average power during each pulse is $0.6W$. Then

• A. The energy of each photon is $3.048\times {10}^{19}J$
• B. The energy content in each pulse is $12mJ$
• C. The number of photons in each pulse is nearly $4\times {10}^{15}$
• D. The energy of each photon is nearly $1.9eV$

Answer 1

Answer: (B), (D)

The energy content in each pulse is $12mJ$
The energy of each photon is nearly $1.9eV$

Energy of a photon is given by:

$E=\frac{hc}{\lambda }=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{652\times {10}^{-9}}$

$E=3.048\times {10}^{-19}J$ Thus, option A is incorrect.
Energy content in each pulse is given by
$E_p=Power\times 20\times {10}^{-3}=12mJ$ Thus, option B is correct.
Number of photons can be obtained using

$n=\frac{12\times {10}^{-3}}{3.048\times {10}^{-19}}=3.94\times {10}^{16}$ Thus, option C is also incorrect.

Energy of each photon in eV is given as
$E=3.048\times {10}^{-19}\times 6.24\times {10}^{18}eV=1.9eV$ Thus, option D is correct.