Question

A laser generates a pulse at 450 nm containing 2.75 × 1015 photons. What is the energy of the pulse?

Answer 1

Given:

lambda = 4.5*10^-7 m

Find energy of 1 photon first

use:

E = h*c/lambda

=(6.626*10^-34 J.s)*(3.0*10^8 m/s)/(4.5*10^-7 m)

= 4.417*10^-19 J

This is energy of 1 photon

Energy of 1 mol = energy of 1 photon * Avogadro's number

= 4.417*10^-19*6.022*10^23 J/mol

= 2.66*10^5 J/mol

= 2.66*10^2 KJ/mol

use:

number of mol = number of photons / Avogadro’s number

number of mol = 2.75*10^15 / 6.022*10^23 mol

number of mol = 4.567*10^-9 mol

Use:

Energy of n mol = n * energy of one mol

Energy of 4.567*10^-9 mol = 4.567*10^-9 * 2.66*10^2

= 1.22*10^-6 KJ

= 1.22*10^-3 J

Answer: 1.22*10^-3 J