The ellipse has an important property that is used in the reflection of light and sound waves. Any light or signal that starts at one focus will be reflected to the other focus. This principle is used in lithotripsy, a medical procedure of treating kidney stones at one focus. High-energy shock waves generated at the other focus are concentrated on the stone, pulverizing it. In addition, each planet travels around the sun in an elliptical orbit with the sun at one foci. The orbits of the moon and of artificial satellites of the earth are also elliptical as are the paths of comets in permanent orbit around the sun.