What Does Internal Reflection Mean?

When light travels from water and glass into air, it is refracted away from the normal. If the angle of incidence is made large enough there will come a time when the light is refracted away from the normal so that it travels along the surface of the water or glass.

For all the angles of incidence greater than the critical angle, the refracted rays are reflected back into the water or glass, that is they do not leave the medium. The light is said to be totally internally reflected. The fact that light undergoes total internal reflection under certain conditions gives rise to, the use of prisms in the manufacture of the optical instruments such as periscope and binoculars, mirages and what is often called a 'fish-eye' view. A prism is a glass block whose sides are inclined to each other and whose edges are parallel. The triangular prism commonly used in optics is right-angled, with the other two. Light rays may enter the prism through any one of its three surfaces, and if the angle of incidence is greater than the critical angle of glass, total internal reflection occurs.