How many photons per second emanate from a 50 mW laser of 640 nm ?

Explanation

Given data `:`
Wavelength of the laser = 640 nm
`lambda = 640 xx 10^(-9) m`
Power of laser P = 50 mw ` = 50 xx 10^(-3) w `
To find `:`
No. of photons emitted per second N =?
The energy of a photon E `= hv = ( hc)/( lambda ) `
c- velocity of light `= 3 xx 10^(8) ms^(-1)`
h - planck constant ` = 6.626 xx 10^(-34) Js`.
P = no. of photons emitted per second `xx` energy of one photon.
P =NE
N = `( P )/( E )`
` = ( 50 xx 10^(-3) xx 640 xx 10^(-9))/( 6.626 xx 10^(-34) xx 3 xx 10^(8)) = ( 32000 xx 10^(-12))/( 19.878 xx 10^(-26))`
`N = 1609.8 xx 10^(14) s^(-1)`
No. of protons emitted per second `= 1.61 xx 10^(17) s^(-1)`