is there a theorem which describes why this creates a right angle?

Question

> draw circle A
> draw smaller circle B such that it passes through center of circle A and intersects circle A at two points (E1 & E2)
> draw a diameter of circle A that passes through center point of circle B; label points on edge of circle A: I=point that lies inside circle B, F=point that does not
> now when you draw two straight lines IE1 and FE1, you get a right angle

WHY?! I want to understand why this is! Can someone point to a theorem where I can explore this idea in greater depth? I have searched and searched with the search terms I can think to use and I keep getting information about orthogonal circles, which is obviously not what I'm looking for.

see the image here: https://i.stack.imgur.com/xjfe8.png

Answer 1

Yes any angle subtended by a diameter is a right angle.