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A bulb emits light of $\lambda=4500A^{\circ}$ .The bulb is rated as 150watt and 8% of the energy is emitted as light.How many photons are emitted by the bulb per second?

$\begin{array}{1 1}(a)\;37.5\times 10^{18}&(b)\;28.6\times 10^{12}\\(c)\;27.2\times 10^{18}&(d)\;32.3\times 10^{14}\end{array}$

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Energy of one photon =$\large\frac{hc}{\lambda}$
$\Rightarrow \large\frac{6.625\times 10^{-34}\times 3\times 10^8}{4500\times 10^{-10}}$
$\Rightarrow 4.42\times 10^{-19}J$
Energy emitted by bulb=$150\times \large\frac{8}{100}$J
$\therefore n\times 4.42\times 10^{-19}=150\times \large\frac{8}{100}$
(n is no of photon)
$n=27.2\times 10^{18}$
Hence (c) is the correct answer.
answered Jan 15, 2014 by sreemathi.v

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