Total Internal Reflection
Suppose a beam of light passes from glass to air. Air has a lower index of refraction, so
the emerging beam is bent away from the normal. As the angle of incidence is increased,
the beam in air moves farther from the normal until it makes a 90-degree angle with the
normal and grazes the surface. The angle of incidence when this occurs is known as
the critical angle.
Increasing the angle incidence still further results in no
light penetrating the glass-air boundary; instead, all the light is reflected back into
the glass. This is known as total internal reflection, an effective way of reflecting
light without using a mirror. (One disadvantage of mirrors is that they have two
reflecting surfaces, resulting in a faint "ghost," or double image.) Total
reflecting prisms are used in fine cameras, binoculars, and other optical instruments to
change the direction of light. Light may be "transported" by transparent fibers
and by glass or plastic rods; whether straight or curved, the light stays inside them
because of successive internal reflections.
 |
i > c
The angle of incidence i is much smaller
than the critical angle c; thus light is refracted. |
 |
i = c
The angle of refraction here is 90 degrees. |
 |
i > c
Total internal reflection occurs.
The angle of incidence,i, is bigger than
the critical angle, c.
Light travels from a denser medium to a
less dense medium. |
