Total internal reflection – slow to fast
When light travels from a medium such as glass to air, total internal reflection can occur.
Remember:
Fast to slow, towards normal line
Slow to fast, away from normal line
Glass to air is slow to fast, therefore:
It is at the point where `theta\ \o\u\t=90^circ` to the normal line that `theta\ \i\n` is at the critical angle.
The critical angle `theta\ \i\n` occurs when the light, `theta\ \o\u\t`, travels along the boundary between the two media.
The critical angle only occurs when travelling from a slow medium to a fast medium.
As the angle of incidence is increased the angle of refraction increases.
At a certain angle `(theta\ \i\n)` the light will refract along the boundary. This is the critical angle for glass to air. It's worth knowing that `(theta\ \i\n)` is about 42 degrees for glass to air but it varies according to the material.
| Water to air | `~~50^circ` | ||
| Glass to air | `~~42^circ` | ||
| Diamond to air | `~~24^circ` |
Past this point "total internal reflection" occurs.
It is not just a part of it that is reflected but all of the light ray gets reflected. This is why they call it total internal reflection.
Total internal reflection occurs when the angle of the light ray going in at angle `(theta\ \i\n)` is greater than the critical angle.
Total internal reflection `theta\ \i\n\>` critical angle
