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Question

# Calculate the number of photons emitted per second by a 10 W sodium vapour lamp. Assume that 60% of the consumed energy is converted into light. Wavelength of sodium light is 662 nm. (Given that h=6.62×10−34 J s)

A
2×1019
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B
3×1019
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C
2×1020
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D
3×1020
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Solution

## The correct option is A 2×1019We 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 60% of the consumed energy is converted into light. So energy used =6 J Again, energy (E)=nhcλ=n×6.62×10−34×3×108662×10−9 (Where n = number of photons) So 6=n×6.62×10−34×3×108662×10−9 ⇒n=6×662×10−96.62×10−34×3×108 ⇒n=2×1019

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