A bulb emits light of $\lambda =4500A°.$  The bulb is rated as 100 watt and 10% of the energy is emitted as light. How many photons are emitted by the bulb per sec.

$$\begin{gathered} E=\frac{hc}{\lambda }=\frac{6.625\times {10}^{-34}\times 3\times {10}^8}{4500\times {10}^{-10}}J =4.42\times {10}^{-19}J \\ Total energy emitted by bulb =\cancel{100}\times \frac{10}{\cancel{100}}=n\times E \\ 10=4.42\times {10}^{-19}\times E E=\frac{1}{4.42}\times 10\times {10}^{+19} \\ =0.2262\times {10}^{20} =22.62\times {10}^{18} \end{gathered}$$