In optics, light focused by reflection from or refraction through a curved object. For example, when a magnifying glass is used to burn a piece of paper, the intense point of focused light at the paper’s surface forms a caustic. Ray tracing algorithms in computer graphics typically have difficulty correctly detecting and handling this phenomenon. An example might be the form of the sunlight on the bottom of a swimming pool when there are ripples on the surface of the water that focus the light into caustics.