Specular reflection is a mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. In the diagram to the right, a light ray is coming from the point P, and meets at the mirror at point O. The ray reflects at the point O outwards in the same medium towards point Q. Reflection is measured relative to a line through point O that is at 90˚ to the mirror. The angles made by the light ray (shown in the diagram as <math>\theta _i</math> and <math>\theta _r</math>) must be equal. Both angles have to be equal whenever reflection occurs. This is known as the "law of reflection".