00:01
So let's say we have an atom with a mass m, and let's say it's at rest, and it either emits or absorbs a photon.
00:09
So the photon has an energy we'll just call hf, or maybe h new.
00:15
So due to the conservation of energy, our initial, for one, let's look at the initial momentum.
00:20
The initial momentum is zero, and so the final momentum has to be zero.
00:24
And so that final momentum is going to be like m times the speed, the recoil speed of the atom, minus like h new, or better yet, we'll put it h new minus mv.
00:37
And this has to be equal to zero, of course.
00:39
So that tells us like, sorry, h new over c.
00:42
That tells us that h new is like mv times c or better yet.
00:48
Let's write it like this...