Focus, in Physics, is a point of concentration of waves of air, water, ether, or other medium capable of transmitting wave motion. The term is more exclusively applied to waves of light and heat, the familiar instance of the concentration of the sun's rays to a point by means of a burning-glass showing at once a focus of light and of heat. In optics a focus is real if the waves actually do reach the point; it is virtual if they only seem to proceed from or towards the point; it is a principal focus if it is the point to which plane-fronted waves may be directed by suitable reflection or refraction. An instrument for determining the positions of the principal foci of lenses is called a focometer; one for "focussing" or bringing the clearest image on the ground-glass of a photographic camera, is called afocimeter. The distance from a lens or mirror to its principal focus is called its focal length. In geometry the focus is a special point in relation to the conic sections, from which the distances of points on the curves may be conveniently measured.