Pastry cutters and caustics

It's dismal out, so I got out the pastry cutters and arranged them like an unfurling bracken frond to remind me of spring, and let the light do the rest. The curves inside formed by the light rays are called caustics; these particular ones are half nephroids (with blemishes because the cutters aren't perfect circles). I call them teacup curves, because you often notice them on the surface of a cup of tea when you're drinking one in the sunshine. Isn't maths wonderful!