Question

a he-ne laser emits a red light having a wavelength of 632.8nm and a power of 0.5mw. how many photons are emitted each second?

Answer 1

λ = wavelength of red light = 632.8 nm = 632.8 x 10⁻⁹ m

P = power of laser =  0.5 mW = 0.5 x 10⁻³ watt

n = number of photons

energy of "n" photons is given as

E = nhc/λ                      

where

c = speed of light = 3 x 10⁸ m/s

h = plank's constant = 6.63 x 10⁻³⁴

dividing both side by time "t"

E/t = (n/t)hc/λ      

we know that , energy/time = power , hence

P = (n/t)hc/λ    

inserting the values in the above equation

0.5 x 10⁻³ =  (n/t) (6.63 x 10⁻³⁴) (3 x 10⁸)/(632.8 x 10⁻⁹)

n/t = 1.6 x 10¹⁵ photons/sec

hence  1.6 x 10¹⁵ photons are emitted per second