The angle at which the ray strikes the surface, called the angle of incidence, is equal to the angle at which it bounces off, called the angle of reflection. When light rays reflect from a flat, polished surface, such as a mirror, projections of these reflected rays seem to converge, or come together, behind the mirror, forming an image. When a light ray strikes the surface between two substances, only part of the ray is reflected. Refraction, or the bending of light, occurs when the second substance has a different density from the first, so that the speed of light in the two substances differs. In this case, if the light ray does not enter perpendicular to the second surface, it will change direction at the interface where the two surfaces meet.
Lenses bend, or refract, light.
If the curvature of the eye's lens is faulty, corrective lenses can improve vision. Because of the curvature of a lens's surfaces, different rays of an incident light beam are refracted through different angles. Thus an entire beam of parallel rays can be caused to converge, or come together, at a single point, or can be caused to appear to diverge, or spread apart, from a single point. This point is called the focal point of the lens. If a lens bends the light rays so that they converge, a real image is formed.