Question

A sodium lamp of power $10$W is emitting photons of wavelength $590$ nm. Assuming that $60\%$ of the consumed energy is converted into light, find the number of photons emitted per second by the lamp.

Answer 1

$P=10W$,

E in $1sec=10J$ % used to convert into photon = $60$

Energy used = $6J$

Energy used to take out $1$ photon

$=\frac{hc}{\lambda }=\frac{6.63\times {10}^{-34}\times 3\times {10}^8}{590\times {10}^9}=\frac{6.633}{590}\times {10}^{-17}$

$=\frac{6}{\frac{6.63\times 3}{590}\times {10}^{-17}}$$=\frac{6\times 590}{6.6\times 3}\times {10}^{-17}$

$176.9\times {10}^{17}=1.77\times {10}^{19}$