How many photons are in the laser pulse?
3 Answers

1 photon will have an energy of hc/lambda = (6.63 x 10^34Js x 3 x 10^8m/s)/505 x 10^9m =
3.94 x 10^19J
Therefore the No. of photons that have a total energy of 4.85mJ =
(4.85 x 10^3)/(3.94 x 10^19) = 1.231 x 10^16 photons

IF the wavelength of the laser pulse is 505nm, the energy possessed by a single photon will be given by {hc/wavelength}, where h=Planck’s Constant= 6.62 x 10 power 34 Js, c=Speed of Light = 3 x 10 power 8 m/s. And the wavelength is given. Substitute in the Planck’s equation and find out the energy of a single photon (lets say it to be x J). Now u have the value for the whole pulse. Divide the value (4.85 MJ) with the energy of the single photon and calculate the number of photons!
P.S. The energy of the laser pulse cannot be such a small value (mJ=milli Joule). Check out the question properly…It has to be MJ=Mega Joule. Milli is 10 power 3 and Mega is 10 power 6.

Mj Of Energy