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Study Notes: Refraction and Total Internal Reflection

When light strikes the surface of a glass slab, some of the light is reflected back from the surface and the rest of the light passes into the glass. When light passes from air into the glass slab, it changes direction - it is said to ‘bend’. This is called refraction of the light.



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The reason for the refraction of the light is that the light changes speed (and wavelength) when it passes from one material into another with a different optical density.

When light travels from air (low optical density) into glass (high optical density) the beam is bent towards the normal. The normal is an imaginary line drawn perpendicular (at 90°) to the surface where the incident beam enters the glass.

When the light emerges from the other side of the glass slab, it is refracted again. This time the light is bent away from the normal because it is moving from higher to lower optically dense materials.

When light passes from glass (an optically dense material) into air (low optical density) it is bent away from the normal.

As the light beam passes through the glass slab we see that the emerging beam is still travelling in the same direction as the incident beam (they are parallel) but it has been moved to one side (laterally displaced).

Total internal reflection
When light travels from glass back into air it is refracted away from the normal. Increasing the angle of incidence at this surface increases the angle of refraction and when this exceeds 90° the light does not leave the glass. We say that it is totally internally reflected.

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The angle of incidence at which total internal reflection begins is called the critical angle. The critical angle depends on optical density so it will be different for each material.

At angles greater than the critical angle the light rays are reflected back from the surface.

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