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We all know that when light hits a plane surface, it reflects back.

Light reflection

And when the light hits plane surface, then if incident angle is greater then critical angle then it reflects back into the medium, this phenomenon is called total internal reflection.

Total internal reflection

Now the thing that always confuses me is: what is the difference between simple reflection and total internal reflection? I mean, in both cases light is reflected back in the same medium. So how we differentiate these terms?

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We all know when a light hits a plane surface it reflects back.

Depending on the medium, some fraction of the light reflects back. If we think of a glass-air boundary, we are used to seeing both reflections and transmitted images on a pane of glass.

In total internal reflection none of the incident light can propagate into the other medium. All of the light will either be absorbed or reflected. This makes sense if you calculate the transmission angle based on refraction. The transmission ray is prohibited.

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  • In normal reflection, some amount of light energy is lost, either due to refraction into the second medium, or absorption, etc. whereas in the total internal reflection all light comes back to the first medium.
  • Total internal reflection only occurs when light travels from higher to lower refractive index, whereas reflection occurs on the interface of any two media.
  • There is a critical angle (depending on the refractive indices of the media) above which total internal reflection occurs, no such critical angle for normal reflection.
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  • $\begingroup$ In an absorbing material (e.g. colored glass) TIR can still occur, so the first bullet point is not quite correct. $\endgroup$ – Ruslan Sep 8 '20 at 18:57
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in both cases light is reflected back in the same medium. So how we differentiate these terms?

In the case of simple reflection of an incident beam there's always a transmitted beam. It might propagate over a very short distance in the second medium due to absorption (like when the second medium is e.g. a metal), but it does propagate as the usual sequence of crests and troughs, and this propagation is definitely directed away from the surface (although possibly at an angle).

As you increase the angle of incidence, the refracted beam turns, until at the critical angle of incidence the refracted beam becomes parallel to the surface. This is the moment when light no longer propagates away from the surface, instead propagating along it. This is the point where total internal reflection begins.

As you further increase the incidence angle beyond the critical angle, the transmitted beam becomes an evanescent wave, whose crests move, as in the case of exactly the critical angle, along the surface. The increase of incidence angle now makes the exponentials fall off more rapidly with distance from the surface.

This evanescent wave is the main difference between simple reflection and total internal reflection.

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