# Angle of Reflection Inside A Circle

Note: I have browsed and found solutions to problems involving an external collision (angle of reflection along the tangent outside).

I am making a program which has a ball inside a larger circle, and I want to have it bounce when it collides with the circumference. I can detect the collision, and I have degrees/radians, speeds in the x and y directions, and now I would like to know what angle I should set the ball to after collision to keep it in the circle.

How do I find the angle after the ball reaches the point and bounces?

• The angle of reflection should be equal to the angle of incidence, just as it's drawn in your diagram, $\theta_r = \theta_i$. – Aaron Golden Nov 13 '15 at 0:33
• This is Alhazen's billiard problem. See also. – Lucian Nov 13 '15 at 1:35