# Average wavelength of light emitted by a bulb with metal filament is 120nm. calculate the number of photons emitted by such bulb with 20W in one hour.

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### 1 Answer

This is related to Planck constant. The energy(E) generate by emitting a photon with wavelength `lambda` and velocity c in one second is given by;

E = `(hc)/lambda` where h is the Planck constant.

c = velocity of light = `3xx10^8m/s`

`h = 6.6xx10^(-34)`

`E = (6.6xx10^(-34)xx3xx10^8)/(120xx10^(-9))`

`E = 1.65xx10^(-18)J`

Energy consumed by bulb in one hour `= 20xx3600 J`

Number of photons emittedper hour

`= (20xx3600)/(1.65xx10^(-18))`

`= 4.4xx10^22`

*So `4.4xx10^22` photons will be emitted in one hour.*

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