Mammoth Memory

Total internal reflection – slow to fast

When light travels from a medium such as glass to air, total internal reflection can occur.


Fast Sofa helps you remember

Fast to slow, towards normal line

Slow to fast, away from normal line

Glass to air is slow to fast, therefore:

Light bending away from the normal line as it travels from a slow medium to a fast medium.

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.

Theta not yet at the critical angle.

As the angle of incidence is increased the angle of refraction increases.

As the angle of incidence increases so does the angle of refraction.

When theta reaches the critical angle the light is refracted along the boundary.

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.

When theta is greater than the critical angle 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

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