# Is total internal reflection just a simple reflection?

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

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.

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?

• Does this answer your question? What is the difference between reflection and total internal reflection (TIR)? Sep 8, 2020 at 16:42
• In your first diagram, are you imagining that the medium is a metal? I.e., a mirror? Or do you imagine that it is any material at all, for example window glass? Nov 5, 2021 at 11:36
• @garyp It could be any material. Nov 6, 2021 at 8:04

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.

• 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.
• In an absorbing material (e.g. colored glass) TIR can still occur, so the first bullet point is not quite correct. Sep 8, 2020 at 18:57

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.

No TIR and reflection are different . Reflection is bouncing back of light in same media after striking reflecting surface . But in TIR light is reflected to same media after refraction . We know that when light moves from denser to rarer media it bends away from normal . The angle of incidence in denser media for which angle of refraction in rarer media is 90 degree is known as critical angle . When angle of incidence is greater then critical angle then TIR occurs . Light bends away from normal so much that it enters in 1st media . In order to understand TIR , I suggest you see diagrams

• I hope that was helpful Nov 5, 2021 at 8:27
• What's wrong with the answer? I got considerable downvotes . something to improve Nov 5, 2021 at 16:00
• I'll take a guess. It looks like you are repeating what the OP has already described. Nov 6, 2021 at 14:09