• Under certain circumstances the transparent boundary between one material and one of lower refractive index can act like a perfect mirror to particular incident rays.
• Whenever a wave crosses a boundary between materials there is always some level of reflection.
• Reflections are always partial reflections
• i.e. the energy of the reflected ray is not equal to the energy of the incident ray.
• Even the metal coatings used to make mirrors absorb some incident wave energy.

In AS/A-Level Physics,

• All rays are reflected (bounce) off the internal walls with no energy loss. This is known as Total Internal Reflection.
• Total internal reflection reflects 100% of the light, subsequent absorption by the glass media and/or boundary losses will reduce to partial internal reflection.
• It only occurs under the following conditions:

1. the ray must be travelling from a material of higher Absolute Refractive Index to lower Absolute Refractive Index.
2. The angle on incidence at the boundary is greater than the Critical Angle, θc

In AS/A-Level Physics, the critical angle (θc) is an angle of incidence which produces an angle of refraction of 90^o.

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