When light crosses materials with different refractive indices, the light beam will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is shallower (closer to the boundary) than the critical angle, the angle of incidence where light is refracted so that it travels along the boundary, then the light will stop crossing the boundary altogether and instead totally reflect back internally. This can only occur where light travels from a medium with a higher refractive index to one with a lower refractive index. For example, it will occur when passing from glass to air, but not when passing from air to glass.
Total internal reflection can be demonstrated using a semi-circular glass block. A "ray box" shines a narrow beam of light (a "ray") onto the glass. The semi-circular shape ensures that a ray pointing towards the center of the flat face will hit the surface at right angles. This prevents refraction at the air/glass boundary.
At the glass/air boundary what happens will depend on the angle. Where θc is the critical angle:
- If θ < θc, as with the red ray, the ray will split. Some of the ray will reflect off the boundary, and some will refract as it passes through.
- If θ > θc, as with the blue ray, all of the ray reflects from the boundary. None passes through.
The second situation is total internal reflection.
This physical property makes optical fibres useful, and rainbows and prismatic binoculars possible. It is also what gives diamonds their distinctive sparkle, as diamond has an extremely high refractive index.
An important side effect of total internal reflection is the propagation of an evanescent wave across the boundary surface. This wave may lead to a phenomenon known as frustrated total internal reflection.
Frustrated Total Internal Reflection Edit
While it is true that the creation of an evanescent wave does not affect the conservation of energy under ordinary conditions, i.e. the evanescent wave transmits zero net energy, if a medium with a higher refractive index is placed less than several wavelengths distance from the boundary of the first medium, the strength of the evanescent wave will be large enough to effect a change in the field of the second material. Electrons driven by the field allow energy to flow across the gap and into the second higher refractive index medium.
A common example in everyday use is a beam splitter. A transparent, low refractive index material is sandwiched between two prisms of another material. This allows the beam to "tunnel" through from one prism to the next in a process very similar to quantum tunneling while at the same time altering the direction of the incoming ray.
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