Question

A laser pulse with wavelength 525 nm contains 4.40 mj of energy. How many photons are in the laser pulse

Answer 1

The laser pulse in this question has a wavelength of
`\lambda=524 nm=525* 10^(-9)m`. To solve this problem, we first have to calculate the energy of a single photon in the laser pulse. The equation for calculating the energy of a single photon of an electromagnetic wave is
`E=(hc)/(\lambda)` where
`c` is the speed of light,
`h` is planks constant and
`\lambda` is the wave length of the photons.

For this problem,
`c=3.0* 10^8m/s`,
`h=6.63*10^(-34)J.s` and
`\lambda=525* 10^(-9)m`. We use these values to calculate the energy of the photon as shown below,

`E=(hc)/(\lambda) \\E=((6.63* 10^(32)Js)*(3.0*10^8m/s))/(525* 10^(-9)m) \\E=3.79* 10 ^(-19)J.`

Now that we know the energy for a single photon, we will divide the total energy given by the energy of one photon to get the number of photons in the pulse. The number of photons
`n` is calculated as shown below,

`n=(4.4* 10^(-3)J)/(3.79*10^(-19)J) =1.16* 10^(16)`. There are
`1.16* 10^(16)` photons.