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

Explanation

Power of the bulb = 100 watt `= 100 J s^(-1)`
Energy of one photon, `E = hv = (hc)/(lamda) ((6.626 xx 10^(-34)Js) xx (3 xx 10^(8) ms^(-1)))/(400 xx 10^(-9)m) = 4.969 xx 10^(-19) J`
`:.` Number of photons emitted `= (100 Js^(-1))/(4.969 xx 10^(-19)J) = 2.012 xx 10^(20) s^(-1)`