00:01
For this exercise, we have a photon that creates an electron positron pair, and our job is to calculate the wavelength, the longest wavelength of the photon that can create such a pair.
00:16
The first thing we got to remember here is that the energy of a photon is given by hc over lambda.
00:24
This is the energy of the photon, hc over lambda.
00:27
So if you're looking for the longest wavelength, what we're really looking for is the smallest energy that can create the pair, positron, electron.
00:40
And what is the minimum energy? well, the minimum energy required to create such a pair is going to be the rest energy of the system.
00:50
So we're not going to give it any kinetic energy.
00:52
We're just going to give it enough energy so that it can have mass.
00:56
So what's the rast energy of the system? it's going to be the rast energy of the electron plus the rest energy of the positron.
01:04
The rast energy of the electron is m .e.
01:07
C squared, where m .e is the mass of the electron.
01:10
And so is the rast energy of the positron because they have the same mass.
01:16
So the total rast energy is going to be 2me c squared.
01:20
And at this time, i think it's important to remember.
01:23
I'm going to write it up here in blue...