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Question

Calculate the number of photons emitted per second by a 10 W sodium vapour lamp. Assume that 50% of the consumed energy is converted into light. Wavelength of sodium light = 600 nm. Take h=6.6×10−34 Js and c=3×108ms−1

A
15.15×1018
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B
25.25×1018
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C
2.52×1019
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D
1.51×1018
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Solution

The correct option is A 15.15×1018
We know, power is the energy transferred or converted per unit time.

Since power is given as 10 W,
so energy consumed in 1 sec is = 10 J

But, given that 50 % of the consumed energy is converted into light.
So energy used = 5 J
Again, energy (E)=nhcλ=n×6.6×1034×3×108600×109
(Where n = number of photons)

So 5=n×6.6×1034×3×108600×109
n=5×600×1096.6×1034×3×108
n=15.15×1018

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