Photons of wavelength $660\text{nm}$ are emitted from a $60$  watt lamp. What is the number of photons emitted per second?

Wavelength $\lambda =660\text{nm=660}\times {\text{10}}^{-9}\text{m}$

Power $\text{P=60}\text{watts}$

Planks constant $\text{h=6}\text{.62}\times {\text{10}}^{-34}\text{J}$

Velocity of light $C=3\times {10}^8{\text{ms}}^{\text{-1}}$

Number of photon per second $\frac{n}{t}=?$

  $\frac{\text{n}}{\text{t}}=\frac{\text{pλ}}{\text{hc}}$              $\left(\because \text{P}=\frac{\text{n}}{\text{t}}\frac{\text{hc}}{\text{λ}}\right)$

$=\frac{60\times 660\times {10}^{-9}}{6.62\times {10}^{-34}\times 3\times {10}^8}$

$=2\times {10}^{20}$