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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 = 590 nm

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Solution

Given:
Power of the sodium vapour lamp, P = 10 W
Wavelength of sodium light, λ = 590 nm
Electric energy consumed by the bulb in one second = 10 J
Amount of energy converted into light = 60 %
∴ Energy converted into light = 60100×10=6 J
Energy needed to emit a photon from the sodium atom,
E'=hcλE'=6.63×10-34×3×108590×10-9E'=6.63×3590×10-17 J

Number of photons emitted,
n=66.63×3590×10-17n=6×5906.63×3×1017
n =
1.77 × 1019

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