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Q.

A bulb of 40 W is producing a light of wavelength 620 run with 80% of efficiency, then the number of photons emitted by the bulb in 20 seconds are :

1eV=1.6×10−19J, hc=12400eV

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a

1021

b

2×1018

c

1018

d

2×1021

answer is D.

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Detailed Solution

Number of photons emitted by the bulb in 20 seconds are 

40×0.81.6×10−19×20=N×1240620 ⇒N=2×1021

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