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# A 100W power source emits green light at a wavelength $\lambda=5000\overset{\circ}{A}$. How many photons per minute are emitted by the source?

$\begin{array}{1 1}(a)\;1.5\times 10^{22}&(b)\;2\times 10^{21}\\(c)\;3\times 10^{24}&(d)\;2.5\times 10^{21}\end{array}$

Energy given out by the source per sec = Power (P)
$\Rightarrow$ Energy given by source in t sec=$P\times t$
As $\lambda=5000\overset{\circ}{A}$,the energy per photon of green light is E/photon=$h\nu=\large\frac{hc}{\lambda}$
Number of photons (n) emitted in time t sec is given by
$n=\large\frac{P\;t}{\Large\frac{h\;c}{\lambda}}=\frac{P\;t\;\lambda}{h\;c}$
$\Rightarrow n=\large\frac{100\times60\times5000\times 10^{-10}}{6.63\times 10^{-34}\times3\times 10^8}$$= 1.5\times 10^{22}$
Hence (a) is the correct answer.
edited Mar 25, 2014