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Q.

A bulb of 40W is producing a light of λ 620nm with 80% of efficiency then the number of photons emitted by the bulb in 20 sec [1ev = 1.6×10–19J, hc=12400ev A0]

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a

2 × 1018

b

1021

c

2 × 1021

d

1018

answer is D.

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Detailed Solution

power=nhcλt            =40×80100              =n×6.62×10-34×3×108620×10-9×20         n     =2×1021

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