Question

If 5% of the energy supplied to a bulb is radiated as visible light, the number of visible quanta emitted per second by a 100W bullb, assuming the wavelength of visible light to be $5.6\times {10}^{-5}cm$, is

• A. $1.4\times {10}^{19}$
• B. $1.4\times {10}^{20}$
• C. $2\times {10}^{19}$
• D. $2\times {10}^{20}$

Answer 1

The correct option is A $1.4\times {10}^{19}$

Energy of single photon = $\frac{hc}{\lambda }$

If there are $n$ photons, total energy = $\frac{nhc}{\lambda }$

Total energy emitted =$0.05\times 100W=5W/s$
Equating

$\frac{nhc}{\lambda }=5$

$n=\frac{5\lambda }{hc}$

Substitute values

$n=\frac{5\times 5.6\times {10}^{-7}}{6.26\times {10}^{-34}\times (3\times {10}^8)}$

$n=1.4\times {10}^{19}$