Total internal reflection:

It is an interesting case of refraction when a light ray travels from a dense medium to rare medium. The light ray can also bend so much that it never goes beyond the boundary between the two media. The case of refraction is called total internal reflection.


How it happens:

Total internal reflection can happen only when a beam of light travelling through a dense medium cross the interface with a rarer medium.

For example, through a glass piece to air, when such a beam reaches an interface it makes an angle (known as the angle of incidence) with the perpendicular at that point. When the beam exists the interface into the rarer medium, it makes a larger angle (known as the angle of refraction) with the same perpendicular.


As the angle of incidence increases, so does the angle of refraction. There is one value of the angle of incidence for which the angle of refraction is 90° and the emerging ray is tangential to the interface. This is called the critical angle.


For all the angles of incidence greater than the critical angle; the incident ray will not emerge into the rarer medium at all. Instead, it gets reflected back into the denser medium itself. This phenomenon is known as total internal reflection.


Rainbow is the result of this phenomenon.

Conditions for total internal reflection:

  1. The light ray should travel from denser medium to rarer medium.
  2. The angle of incidence should be more than critical angle or 90°.

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