Q 12-11-177JEE MainJEE Main 2017 (9 Apr)Easy
A laser light of wavelength $660$ nm is used to weld Retina detachment. If a laser pulse of width $60$ ms and power $0.5$ kW is used, the approximate number of photons in the pulse are: (Take Planck's constant, $h = 6.62\times10^{-34}$ J s)
Answer: (C) $10^{20}$
Energy of the pulse: $E = Pt = 500\times0.060 = 30$ J.
Energy of one photon:
$$\frac{hc}{\lambda} = \frac{6.62\times10^{-34}\times3\times10^8}{660\times10^{-9}} \approx 3.0\times10^{-19}\ \text{J}$$
$$N = \frac{30}{3.0\times10^{-19}} = 10^{20}$$
Solution by Sreeraj P, M.Sc Physics