# About compton's effect of shift in wavelengths

1. Jun 17, 2014

### Esfand Yar Ali

I want to know that ,I have read in many books regarding compton's effect that a photon collides with an electron and this collision is a head-on collision,but if this is a head-on collision then why both these particles are deflected by making a respective angle,don't they should move forward linearly?plz answer this question

2. Jun 17, 2014

### ZapperZ

Staff Emeritus
Please provide references that stated that it is a "head-on" collision.

Zz.

3. Jun 17, 2014

### Matterwave

As both the photon and the electron are point particles in standard QFT, it would be basically undefined what "head on" means. Two points can't really hit "head on" since they have no spatial dimension.

There are different ways you can think about the problem, but two billiard balls hitting right head on is probably not a good way. In semi-classical quantum mechanics (Compton's original derivation uses a semi-classical approach, basically appealing to quantum mechanics only in postulating a photon), from which the photon is modeled as an object with momentum p=h/λ, one should probably just think of the two particles as colliding approximately head on, allowing for the photon to scatter at random angles and imparting an angle dependent amount of momentum to the electron.

Another way to think about it, supported by the Feynman diagram for this process, is to think about the electron actually absorbing the photon, and then re-emitting it in a different (random) direction. If this is easier for you to visualize, you can think of it this way too.

Really, quantum mechanics doesn't say much for the "moment of impact" since all the tests we can do are scattering events where we see the initial state (2 particles going in) and the final state (2 particles coming out) but we can't really ever observe the "moment of impact" itself.

4. Jun 18, 2014

### Esfand Yar Ali

you can look at 'Halliday-Resnick-Walker's Physics'

5. Jun 18, 2014

### Esfand Yar Ali

Quantum Physics is as much interesting as reletivity physics and cosmology!

6. Jun 18, 2014

### ZapperZ

Staff Emeritus
Exactly where? Cite the page number and the passage where this "head-on" collision was stated.

In any case, based on the responses that you had already been given, do you still think that this is a "head-on" collision?

Zz.

7. Jun 18, 2014

### Staff: Mentor

Natural language is not nearly as precise as mathematics, and even very carefully edited textbooks sometimes slip up and use language that can be misinterpreted - especially when they're counting on the student working through the accompanying mathematical treatment to clear up any confusion.

8. Jun 20, 2014

### Esfand Yar Ali

now my concepts are clearing a little bit,but I need some more explainations,well thanks for your support!