# Total Internal Reflection

When a wave hits a boundary with a medium that it can travel faster in (e.g. light going from glass into air) it will be refracted through a larger angle than its angle of incidence.

The bigger the angle of incidence gets the bigger the angle of refraction will get.

This has a limit though! The angle of refraction cannot get bigger than 90o.

A special name is given to the angle of incidence that produces an angle of refraction of 90o.

It is called the critical angle.

If the angle of incidence gets any bigger refraction is not possible and all the light is then reflected.

Total Internal Reflection causes 100% reflection.

In no other situation in nature does this occur, so it is unique and very useful as it is 100% efficient at transfering the light energy.

Click on the graphic for a larger version

If you observe carefully when you carry out an experiment into refraction you will notice that light is not just refracted but that some of it is reflected as well. The larger the angle of incidence the more of the light is reflected and less of it passes into the other medium. You therefore get weak reflection and good transmission into the other medium (refraction) with a small angle of incidence and stronger reflection and less transmission as the angle of incidence gets bigger.

When it reaches the critical angle you get NO transmission of the light into the other medium, it is all reflected.

That is why it is called TOTAL internal reflection, and that is why you must always include that word TOTAL when it applies.

### Total internal reflection occurs when:

a ray of light is incident upon a boundary with an optically rarer medium (one that makes it speed up) and

the angle of incidence is greater than the critical angle. >

(You must mention BOTH points when asked to explain the conditions under which TIR will be observed).

The critical angle is the angle of incidence that produces an angle of refraction of 90o.