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Questions  >>  SAT  >>  Chemistry  >>  Atomic Structure
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Q)

A bulb emits light of wavelength $4500\;A$ . The bulb is rated as $150\;walt$ and $8\%$ of the energy is emitted as light. How many photons are emitted by the bulb per second .

$\begin{array}{1 1} (a)\; 2.715 \times 10^{19} \\(b)\; 1.715 \times 10^{19} \\ (c)\; 3.715 \times 10^{19} \\ (d)\; 4.715 \times 10^{19} \end{array} $

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A)
Solution :
A 150 watt bulb emits 150 J of the energy
The energy emitted by the bulb as light
$\qquad= 150 \times \large\frac{8}{100}$$=12\;J$
Suppose the bulb emits n photons per second
$E= nhv=\large\frac{nhc}{\lambda}$
$n=\large\frac{E \times \lambda}{c \times h}$
$\qquad= \large\frac{12 J \times 4500 \times 10^{-10}\;m}{3 \times 10^8\;ms^{-1} \times 6.63 \times 10^{-34}\;Js}$
$\qquad= 2.715 \times 10^{19}$
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