High School Alpha particle in a force field depiction

[hawkslime]

TL;DR

Can someone give me an explanation of the below picture, please?

Hello, I was glazing through what I would consider an advanced physics textbook and I saw this image. It is a schematic picture of an alpha-particle in a field of an atom.

Now, can someone get me started on what (and why and how) is going on in it? Especially with the fraction with pi.

 

[Baluncore]

Welcome to PF.
Can you please give a reference; Title; Author; ISBN; Page number.

 

[mfb]

The alpha particle approaches a positively charged nucleus, it is repelled and its path is deflected by an angle $\theta$. The rest is just geometry. The sum of the two angles with the fraction is $\pi-\theta$ because these two plus the $\theta$ angle must add up to $\pi$. The dashed line is going through the point of closest approach. The particle trajectory is symmetric with respect to that line, so both sides of it have half of the $\pi-\theta$ angle.

 

[hawkslime]

The alpha particle approaches a positively charged nucleus, it is repelled and its path is deflected by an angle $\theta$. The rest is just geometry. The sum of the two angles with the fraction is $\pi-\theta$ because these two plus the $\theta$ angle must add up to $\pi$. The dashed line is going through the point of closest approach. The particle trajectory is symmetric with respect to that line, so both sides of it have half of the $\pi-\theta$ angle.

Thank you.