Question

Calculate the number of photons emitted per second by a 10 W sodium vapour lamp. Assume that 60 $\%$ of the consumed energy is converted into light. Wavelength of sodium light = 590 nm.

Answer 1

p = 10 W $\therefore$

E in 1 Sec = 10 J,

$\%$ used to convert into photon

= 60 $\%$

$\therefore$ Energy used = $\frac{60}{100}\times 10=6\text{J}$

Energy used to take out 1 photon

$=\frac{hc}{\lambda }$

$=\frac{6.63\times {10}^{-34}\times 3\times {10}^3}{590\times {10}^{-9}}$

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

No. of photons used

$\text{No. of photons used}$

$=\frac{6}{\frac{6.63\times 3}{590}\times {10}^{-9}}$

$=\frac{6\times 590}{6.63\times 3}\times {10}^{17}=1.77\times {10}^{19}$