Total internal reflection
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Total internal reflection is an optical phenomenon. When light crosses materials with different refractive indices, the light beam will be bent at the boundary surface (i.e., refraction). At a certain angle of incidence (the critical angle θc), the light will stop crossing the boundary but instead reflect back internally at the boundary surface. This occurs only at a high refractive index/low refractive index boundary, not the other way around. For example it will occur when passing from glass to air, but will not occur when passing from air to glass.
Total internal reflection is taught in school science labs using a semi-circular glass block. A ray box shines a light 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.
| θ1 <θc | θ2 >θc |
| the ray will split. Some of the ray will reflect off the boundary, and some will refract as it passes through. |
All of the ray reflects from the boundary. None passes through. |
The colour of the rays in the diagram is there to help distinguish the rays, and is not meant to indicate any colour dependence.
This physical property makes optical fiber useful, and the rainbow in the sky and prismatic binoculars possible.