Number of photons of equal energy emitted per second by a 6 mW laser source operating at 663 nm is ____ . (Given: $$h=6.63\times 10^{-34}J.s\text{ and }c=3\times 10^{8} m/s$$)

Find the number of photons emitted per second by a 6 mW laser at 663 nm.

Power $$P = 6$$ mW $$= 6 \times 10^{-3}$$ W, $$\lambda = 663$$ nm, $$h = 6.63 \times 10^{-34}$$ J.s, $$c = 3 \times 10^8$$ m/s.

$$E = \frac{hc}{\lambda} = \frac{6.63 \times 10^{-34} \times 3 \times 10^8}{663 \times 10^{-9}}$$

$$= \frac{19.89 \times 10^{-26}}{663 \times 10^{-9}} = \frac{19.89}{663} \times 10^{-17}$$

$$= 0.03 \times 10^{-17} = 3 \times 10^{-19}$$ J

$$n = \frac{P}{E} = \frac{6 \times 10^{-3}}{3 \times 10^{-19}} = 2 \times 10^{16}$$

The correct answer is Option 4: $$2 \times 10^{16}$$.