The fastest light can travel is when it is in a vacuum, where it travels at 3x10^8 metres per second.

However, when light travels through another substance or material (another 'medium'), it gets slowed down.

This is similar to how you run fastest through air/ slowed down a bit when running through water/ slowed down a lot when trying to run through mud.

If a medium slows light down a lot, it is said to have a high optical density, whereas a medium that doesn't slow light down very much has a low optical density.

This is measured by the medium's refractive index. A high refractive index means the medium has a high optical density, and vice versa.

The lowest refractive index a medium can have is 1, which is in a vacuum, as it doesn't slow light down. There is no limit on how high it can be.

If light travels at an angle (not 90 or 180 degrees), from a low optical density medium to a high optical density medium, it 'bends' towards the most optically dense medium.

Think of the car analogy:
Think of a car travelling at an angle from a smooth surface (effectively low optical density) to a rough surface (effectively high optical density), e.g. from a road to sand. When one of the wheels of the car hits the rough surface, it gets slowed down, and so turns the car towards the rougher surface. This 'turning' effect of light is known as refraction, and has many uses from lenses to optical fibres.

Here are some key phrases to help describe a diagram:
-The line where two mediums meet is known as the boundary.

-The line perpendicular (at 90 degrees) to the boundary is called the normal.

-Incident light is the light that is coming in to hit the boundary.

-Refracted light is the light that has been refracted, and is moving away from the boundary.

-The angle that incident light makes with the normal is called the angle of incidence.

-The angle that refracted light makes with the normal is called the angle of refraction.

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