Question

a) A 100 watt bulb emits monochromatic light of wavelength 400nm. Calculate

number of photon.
​

Answer 1

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Power of the bulb = 100 watt = 100J/s

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Energy of one photon is E = hν = hc/λ

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where, 

h = 6.626 × $\sf{10^{-34}}$ Js, c = 3 × 10⁸ m/s

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Given,

 λ = 400nm = 400 × $\sf{10^{-9}}$m

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By putting the values, we get

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$\implies\sf{\frac{6.626~×~10^{-34}~×~3~×~10⁸}{400~×~10^{-9}}}$

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E = 4.969×$\sf{10^{-19}}$J

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Number of photons emitted in 1 sec × energy of one photon = power

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$\implies\sf{n~×~4.969~×~10^{-19}~=~100}$

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$\implies\sf{n~=~\frac{100}{4.969~×~10^{-19}}}$

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$\underline{\sf{\orange{n~=~ 2.012×10²⁰`photons~ per ~sec}}}$

So, option C is correct

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