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Question

A 100 Watt bulb emits monochromatic light of wavelength 400 nm. Calculate the number of photons emitted per second by the bulb.


A

4 × 1020 s1

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B

2 × 1020 s1

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C

8 × 1020 s1

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D

1× 1020 s1

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Solution

The correct option is B

2 × 1020 s1


We know that the wave velocity (in this case - the speed of light in vacuum)

c = νλ ν=cλ and, by Planck's Theory.

E= hν E = hcλ

Power of the bulb = 100 watt.

= 100 J s1

Energy of one photon is calculated as:

E = hν = hcλ

= 6.626×1034Js×3×108ms1400×109m

= 4.969 × 1019J

Number of photons emitted (per second):

= 100Js14.969×1019J= 2.012 × 1020 s1


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