Question

The number of photons of wavelength $540nm$ emitted per second by an electric bulb of power $100W$ is: (taking $h=6\times {10}^{-34}Js$)

• A. $100$
• B. $3\times {10}^{20}$
• C. $3\times {10}^{18}$
• D. $1000$

Answer 1

The correct option is B $3\times {10}^{20}$

$100W=100J/s$

$p\times t=E$

$E=100J$

$c=v\lambda$

$v=c/\lambda =3\times {10}^8/540\times {10}^{-9}=5.5\times {10}^{14}{s}^{-1}$

$E=hv$

$=6.626\times {10}^{-34}\times 5.5\times {10}^{14}=3.63\times {10}^{-19}$

number of photon = total energy / energy of photon

$n=100/3.63\times {10}^{-19}=3\times {10}^{20}$