A 2 mW laser operates at a wavelength of 500 nm . The number of photons that will be emitted per second is: [Given Planck's constant $\mathrm { h } = 6.6 \times 10 ^ { - 34 } \mathrm {~J} \mathrm {~s}$, speed of light $\mathrm { c } = 3.0 \times 10 ^ { 8 } \mathrm {~m} / \mathrm { s }$ ]\\
(1) $1.5 \times 10 ^ { 16 }$\\
(2) $5 \times 10 ^ { 15 }$\\
(3) $2 \times 10 ^ { 16 }$\\
(4) $1 \times 10 ^ { 16 }$