A laser emits a pulse of light that lasts $10 \mathrm {~ns}$. The light has a wavelength of $690 \mathrm {~nm}$, and each pulse has an energy of $480 \mathrm {~mJ}$. How many photons are emitted in each pulse? $\left( c = 3.0 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s } , h = \right.$ $\left. 6.626 \times 10 ^ { - 34 } \mathrm {~J} \cdot \mathrm { s } \right)$
A) $1.7 \times 10 ^ { 18 }$
B) $3.1 \times 10 ^ { 43 }$
C) $2.1 \times 10 ^ { 27 }$
D) $2.6 \times 10 ^ { 37 }$

Question

Quizplus · Accepted Answer

The energy of a single photon is given by $E = \frac{hc}{\lambda}$, where $h$ is Planck's constant, $c$ is the speed of light, and $\lambda$ is the wavelength. Substituting the given values, $E = \frac{6.626 \times 10^{-34} \cdot 3.0 \times 10^8}{690 \times 10^{-9}}$. The total number of photons is the total energy divided by the energy per photon, $N = \frac{480 \times 10^{-3}}{E}$, which gives approximately $1.7 \times 10^{18}$ photons.

A Laser Emits a Pulse of Light That Lasts 10 ns10 \mathrm {~ns}10 ns

A Laser Emits a Pulse of Light That Lasts $10 \mathrm {~ns}$