Question

The power output of a laser is measured by its wattage, the number of joules of energy it radiates per second (1 W = 1 J s-1). A 300-mW krypton laser produces a beam of red light with a wavelength of 676.4 nm (6.764×10-7 m). (a) Calculate the energy carried by each photon. (b) Calculate the number of photons emitted by the laser per second.

Answer 1

Explanation:

It is given that,

Wavelength of red light, $\lambda=676.4\ nm=676.4\times 10^{-9}\ m$

Power of the laser, $P=300\ mW=0.3\ W$

(a) The energy carried by each photon is given by :

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

$E=\dfrac{6.63\times 10^{-34}\times 3\times 10^8}{676.4\times 10^{-9}}$  

$E=2.94\times 10^{-19}\ J$

(b) Let n is the number of photons emitted by the laser per second. It can be calculated as :

$n=\dfrac{Power}{Energy\ of\ one\ photon}$

$n=\dfrac{0.3}{2.94\times 10^{-19}}$      

$n=1.02\times 10^{18}\ photon/s$

Hence, this is the required solution.