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

Multiple Choice

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) $2.6 \times 10 ^ { 37 }$