Refraction is the basis of the operation of lenses. Light rays bend as they enter and leave the convex lens, as shown in the drawing.
The angles of incidence (i) and refraction (R) specify the path of the light as it moves from one substance into another, as shown in the drawing.
When light moves from air into gelatin, the equation relating the angles i and R can be written as
sin(i) = n(gelatin) X sin(R)

The quantity n(gelatin) is the index of refraction of gelatin. (The value of the index of refraction of air is 1.000, so it is not shown in the equation above) Going from air into gelatin, light bends toward the normal. Consequently, the ratio sin(i)/sin(R) is greater than one (except for the special case of i = 0°). Going from gelatin into air, light bends away from the normal. The angle of refraction is greater than the angle of incidence The critical angle occurs as light leaves the medium with the higher index of refraction (in this activity, moving from gelatin into air). Look at the diagram. It shows how the angle of refraction changes as the angle of incidence, in the gelatin, increases from zero.
Eventually the angle of refraction reaches 90°.

The corresponding angle of incidence is called the critical angle. What happens when the angle of incidence exceeds the critical angle? The light cannot be refracted, because the angle of refraction would be greater than 90°. It turns out that all the light is reflected back into the medium with the higher index of refraction. This effect is called total internal reflection. Although there appears to be an abrupt change at the critical angle, internal reflection begins considerably before reaching the critical angle and becoming total. Likewise, the intensity of the refracted beam diminishes progressively as the critical angle is approached, and this effect can make the critical angle challenging to measure.
Total internal reflection is the basis of the operation of the light pipe. A light beam enters a small-diameter, flexible glass rod. When the beam hits the wall of the pipe, total internal reflection occurs and the beam reflects entirely within the pipe. In this way, light signals can carry information through a system of bundled glass rods called fiber optic cables.