A bulb emits light of wavelength `4500` Å .The bulb is rated as `150` W and `8%` of the energy is emmitted as light. How many photons are emitted by the bulb per second?

Explanation

Electrical energy in joules = Power in watts `xx` Time in seconds
Thus 150 watt = 150 joules of energy emitted per second
`:.` Energy emitted as light `= (8)/(100) xx 150 = 12J`
`E = n hv = nh (c)/(lamda) " " :. n = (E xx lamda)/(h xx c) = ((12J) xx (4500 xx 10^(-10) m))/((6.626 xx 10^(-34) Js) xx (3 xx 10^(8) ms^(-1))) = 2.717 xx 10^(19)`