reflection through the unit circle
You can move the circle around by dragging A or B; the point C moves on the circle.
"Show line" shows a line RS and its reflection (circle R'S'O).
"Show similars" shows the similar triangles to see that the reflection of ABC is indeed a circle (since angle C' must be right)
"Show triangle" shows the triangle ABC and its reflection (arcs A'C', B'C' and A'B')