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Total internal reflection

  Critical angle
When light speeds up as it passes from  from a Slow to a Fast medium         
 (more dense to less dense) , the angle of refraction is bigger than the angle of incidence.
For example, this happens when light passes from water to air or from glass to water.
Angle of incidence and angle of refraction through water and air

The diagram above shows light incident on a water-air interface.

When the angle of refraction is equal to Equation: 90^circ, the angle of incidence is called the critical angleEquation: theta_{c}.

What happens if the angle of incidence is bigger than the critical angle?

As the angle of incidence increases, the angle of refraction gets closer to ninety degrees.

YouTube Video

At any angle of incidence greater than the critical angle, the light  cannot pass through the surface - it is all reflected.

This is called total internal reflection.
Total because all of the energy (light) is reflected
Internal because the energy (light) stays inside the material
Reflection because the light is reflected

Two Requirements for Total Internal Reflection:

Total internal reflection is the phenomenon that involves the reflection of all the incident light off the boundary. Only takes place when both of the following two conditions are met:

  • The light is in the more dense medium and approaching the less dense medium.
  • The angle of incidence is greater than the critical angle.
light travels from glass to air.
Angles of refraction is greater than angle of incidence.

Light travels from glass to air .
Angle of refraction is now 90 degrees 

Light is reflected back into glass.
Angle of reflection is equals angle of incidence.

  • YouTube Video

Optic fibers 
.An optical fiber is a flexible, transparent fiber made by drawing glass (silica) or plastic
 to a diameter slightly thicker than that of a human hair.

 Very little light is absorbed in the glass. Light getting in at one end is totally internally reflected, even when the fibre is bent

A cladding  layer:

s made from material which is less dense to keep light from escaping out of the core of an optical fiber.

YouTube Video

Usage of optical fibers:
An optical fiber is a thin rod of high-quality glass. Light getting in at one end undergoes repeated total internal reflection - 
even when the fiber is bent - and emerges at the other end.

 Communication Endoscope 

Optical fibres have become very important in high-speed communications, such as cable TV and high-speed broadband services.

  An endoscope is an instrument used in medicine to help examine
the inside of  the body, 
which uses bundle of  optical fiber to transmit the image around corners.