Question

Monochromatic light of wavelength 440 nm is produced. The power emitted by light is 18 mW. The number of photons emitted per second by light beam is $(h=6.6\times {10}^{-34}Js)$

• A. $3\times {10}^{16}$
• B. $4\times {10}^{16}$
• C. $3\times {10}^{18}$
• D. $4\times {10}^{18}$

Answer 1

The correct option is B $4\times {10}^{16}$

Given,

$\lambda =440nm$

$h=6.6\times {10}^{-34}Js$

$I=18mW$

The energy of monochromatic light,

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

$E=\frac{6.6\times {10}^{-34}\times 3\times {10}^8}{440\times {10}^{-9}}$

$E=0.045\times {10}^{-17}J$

The number of photon emitted per second by the light beam,

$n=\frac{I}{E}$

$n=\frac{18\times {10}^{-3}}{0.045\times {10}^{-17}}$

$n=4\times {10}^{16}$

The correct option is B