Question

A laser beam of output power ${}^{\prime }{P}^{\prime }$ consists only of wavelength ${}^{\prime }{\lambda }^{\prime }$ . If Planck's constant is ${}^{\prime }{h}^{\prime }$ and the speed of light is ${}^{\prime }{c}^{\prime }$, then the number of photons emitted per second is :

• A. $\frac{hc}{P\lambda }$
• B. $\frac{P\lambda }{h}$
• C. $\frac{P\lambda }{hc}$
• D. $\frac{hc}{P}$

Answer 1

Answer: (C)

$\frac{P\lambda }{hc}$

Power is total energy emitted per unit time $=$ total number of electrons emitted per unit time X energy of each electron
$P=n\times \frac{hc}{\lambda }$
$n=\frac{P\lambda }{hc}$