Total Internal Reflection: Diamonds & Fiber Optics

When light passes from one medium into a second, less dense medium, the light bends away from the normal. At a particular incident angle, the angle of refraction will be 90 degrees, and the refracted ray would travel along the boundary between the two media.

The incident angle at which this occurs is called the critical angle. This angle can be calculated by using Snell's Law:

sin(criticle angle) = (n2/n1)(sin 90 degrees) = (n2/n1)

Where n1 = index of refraction in 1st medium

n2 = index of refraction in 2nd medium

If the angle of incidence is increased beyond the critical angle, the light rays will be totally reflected back into the incident medium. This effect is called total internal reflection. But note that total internal reflection cannot occur if light is traveling from a less dense medium to a denser one.

Diamonds achieve their brilliance partially from total internal reflection. Because diamonds have a high index of refraction (about 2.3), the critical angle for the total internal reflection is only about 25 degrees. Incident light therefore strikes many of the internal surfaces before it strikes one less than 25 degrees and emerges. After many such reflections, the colors in the light are separated, and seen individually.

In the late 20th century, a new, practical application of total reflection was found in the use of fiber optics. If light enters a solid glass or plastic tube obliquely, the light can be totally reflected at the boundary of the tube and, after a number of successive total reflections, emerge from the other end. Glass fibers can be drawn to a very small diameter, coated with a material of lower refractive index, and then assembled into flexible bundles or fused into plates of fibers used to transmit images, telephone calls, video signals, and computer data. The reflective properties of fiber optic strands make fiber optic cables more efficient than cables that transmit signals electrically. The flexible bundles, which can be used to provide illumination as well as to transmit images, are valuable in medical examination, as they can be inserted into various openings.

Information courtesy of:

Optics, Microsoft Encarta Online Encyclopedia 2000 1997-2000 Microsoft Corporation.

Giancoli, D.C. , Physics for Scientists and Engineers 3rd edition, Prentice Hall, New Jersey 2000