Why in Optics total internal reflection happens this way?

[idmond dantes]

why total internal reflection happens only when a beam of light
travels from a medium with higher index of refraction to a medium
with lower index of refraction and not the other way around
(i.e from a medium with lower index of refraction to a medium with a higher one)?

 

[Drakkith]

From wiki:http://en.wikipedia.org/wiki/Total_internal_reflection

When a wave crosses a boundary between different materials with different kinds of refractive indices, the wave will be partially refracted at the boundary surface, and partially reflected. However, if the angle of incidence is greater (i.e. the direction of propagation or ray is closer to being parallel to the boundary) than the critical angle – the angle of incidence at which light is refracted such that it travels along the boundary – then the wave will not cross the boundary and instead be totally reflected back internally. This can only occur when the wave in a medium with a higher refractive index (n1) hits its surface that's in contact with a medium of lower refractive index (n2). For example, it will occur with light hitting air from glass, but not when hitting glass from air.

The key here lies in the fact that the waves are refracted towards the normal of the boundary when passing from a medium with lower to higher refractive index, but are refracted away from the normal of the boundary when passing from a higher refractive index to a lower refractive index.

When a ray of light passes from the lower refractive index to the higher, it is refracted towards the normal. In other words, the angle at which the ray emerges from the boundary is less than what the angle of incidence was before the ray passed through the boundary. So a ray of light entering a medium with a higher refractive index at an angle of 45 degrees is refracted to a lesser angle after passing through, say 30 degrees.

However, if the ray of light is traveling the opposite way and strikes the boundary at 45 degrees it will emerge into the lower refractive index medium at a greater angle, say 60 degrees. If we gradually increase the angle at which strikes the boundary, we will see the angle at which it is refracted increases until the ray is refracted parallel to the boundary. At this angle no refraction can take place and total internal reflection occurs.

Make sense?