• When light travels from an optically less dense medium to an optically more dense medium, it is refracted towards the normal e.g when light travels from air to water as shown in the diagram above. (Here the angle of incidence is larger than the angle of refraction).

• But when light travels from an optically more dense medium to an optically less dense medium, it is refracted away from the normal.

      (Angle of refraction > Angle of incidence) e.g when light travels from water to air.

As the angle of incidence in the denser medium increases the angle of refraction also increases. If this continues until the angle of refraction reaches 90⁰, the angle of incidence is called the critical angle C. A critical angle is defined as the angle of incidence in the denser medium for which the angle of refraction is 90⁰ in the less dense medium.

By the principle of the reversibility of light,

_aη_g= sin90⁰/sin C =1/sin C.

If the angle of incidence exceeds the critical angle, the light undergoes total internal reflection. This reflection obeys all the laws of reflection.

For total internal reflection to occur, two conditions must be satisfied, namely:

• Light must pass from an optically denser medium to a less optically dense medium.
• The angle of incidence in the denser medium must be greater than the critical angle.

Define the term total internal reflection.

State two conditions for total internal reflection to occur.