Question

a sodium lamp of power $10$ W is emmiting phtons 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 $1$ sec = $10J$

used to convert into photon $=60$

Energy used $=6J$

Energy used to take out $1$ photon = $hc/\lambda$ =$6.63\times {10}^{-}34\times 3\times {10}^8/590\times {10}^{-}9$

$=6.633/590\times {10}^{-}17$

No. of photons used $=6/6.63\times 3/590\times {10}^{-}17$

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

$=176.9\times {10}^{1}7$

$=1.77\times {10}^{1}9$