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# An argon laser emits blue light with a wavelength of 488.0nm. How many photon are emitted by this laser in 2.00 sec,operating a power of 515 milliwatts?(one watt = 1J$S^{-1}$)

$(a)\;1.2586\times10^{18}photon\qquad(b)\;2.5286\times10^{10}photon\qquad(c)\;2.5286\times10^{18}photon\qquad(d)\;3\times10^{18}photon$

Power = 15 milliwatts
=0.515 watt=0.515 J/S
$\therefore Energy = 0.515\times2.00$
1.030J obtained in 2 second
E = NhV = $\large\frac{Nhc}{\lambda}$
N = $\large\frac{E\lambda}{hc}$
$=\large\frac{1.030\times488\times10^{-9}}{6.626\times10^{-34}\times3\times10^8}$
$=2.5286\times10^{18}$ photons
Hence anwer is (c)