B Electron's movement after getting hit by a photon that bounces at 90 degrees

[swnsy05]

TL;DR

Electron gets hit by photon, photon bounces off 90 degrees. Why can't electron keep going in a straight path?

I just started studying Quantum physics and i'm learning about Comptonspreading and I have this question in my book that I don't completely understand

So if an electron is standing still, then it gets hit by a photon moving in a straight line into the electron. The photon then bounces off the electron and creates a 90 degree angle from the path it originally had. Why is it not possible that the electron can keep moving straight forward after the collision? It does say that it is not possible in the book but why is it not possible?

 

[PeroK]

It's not possible because of conservation of momentum.

 

[swnsy05]

It's not possible because of conservation of momentum.

So because before the collision there is no momentum in y-direction but after collision there is and that is impossible? Did I get that right? Thanks for your answer by the way, I appreciate it a lot

 

[PeroK]

So because before the collision there is no momentum in y-direction but after collision there is and that is impossible? Did I get that right? Thanks for your answer by the way, I appreciate it a lot

Yes, the momentum of the photon plus the electron after the collision mst equal the momentum of the photon before the collision. The electron must have momentum in the x and y directions in that diagram.

 

[swnsy05]

Yes, the momentum of the photon plus the electron after the collision mst equal the momentum of the photon before the collision. The electron must have momentum in the x and y directions in that diagram.

Okay thank you very much, now I can finally move on from that question. I appreciate the help :)