Question

a bulb rated as 120 watt emits light of wavelength 4000A⁰. if only 10% of energy is emitted as light . how many photons are emitted per second

Answer 1

$$\begin{gathered} E=Nh\nu \\ =\frac{Nhc}{\lambda } \\ E=120W=120J \\ E=120\times \frac{10}{100}J(\sin ce10\%isused) \\ 120\times \frac{10}{100}=\frac{N\times 6.626\times {10}^{-34}\times 3\times {10}^8}{4000\times {10}^{-10}} \\ N=\frac{4000\times {10}^{-10}\times 120\times 10}{6.626\times {10}^{-34}\times 3\times {10}^8\times 100}=2.41\times {10}^{27}photons \end{gathered}$$