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

Explanation

Energy emitted by the bulb = 100 watt = 100 J/s
Energy of one photon
`E = hv = (hc)/lambda`
`lambda = 400 nm = 400 xx 10^(-9) m, h = 6.62 xx 10^(-34) J s`
`c = 3.0 xx 10^8 ms^(-1)`
`:. " " E = ((6.62 xx 10^(-34) J s) xx (3.0 xx 10^(8) ms^(-1)))/(400 xx 10^(-19) J`
No. of photons emitted per sec
`= (100)/(4.965 xx 10^(-19))`
`= 2.01 xx 10^20`