Mathematical curve for reflected light in the bottom of a cup

Factual Questions
1

Whenever you have a cup or glass illuminated from above (but not directly above), much of the light reflects off of the sides of the glass to form a crescent-shaped pattern on the bottom of the cup. It looks somewhat like part of a cardioid, having a cusp pointed toward the center of the bottom surface.

A long time ago, I think that I read somewhere what it is, but now I don’t remember.

I was just curious, seeing them everyday and not knowing what they are.

2

I can’t get behind the mathematics on this one, but there is a name for this reflective/refractive phenomenon: Caustics.

It happens with rings too, if you set one on a flat surface. The wavy light at the bottom of a pool as well.

3

Clinking on a link within the “caustics” article seems to identify that shape as a “nephroid.” Thank you very much.