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
http://encarta.msn.com 1997-2000 Microsoft Corporation.
Giancoli, D.C. , Physics for Scientists and Engineers 3rd edition, Prentice Hall, New Jersey 2000