Refraction and ray diagrams

​​In a nutshell

Refraction is when a wave is deflected at the boundary between two mediums. The speed of a wave is different in different mediums. Refraction can be represented by a ray diagram to show how the direction of the wave changes.

Refraction

Refraction occurs at the boundary between two mediums (materials). It is a specific case of transmission. Transmission is when light passes through a material.

As a wave has different speeds in different mediums, it will travel at a different angle through each medium. The wavelength of a wave also changes in different mediums, but the frequency always stays the same.

The speed of a wave depends on how dense a medium is. The optical density of a medium tells you how quickly a light wave can travel through it. A higher optical density means the light will travel slower. A lower optical density means the light will travel faster.

Example

A wave will travel quicker through air than it does water. This is because water is optically more dense. 

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Ray diagrams

Ray diagrams are useful for showing how a wave travels in different mediums. They can be used to show refraction.

Example

There are three steps to follow when constructing a ray diagram.

Procedure

1.	Draw the boundary of the two mediums and the normal line (dashed line).
2.	Draw the incident ray.
3.	Draw the refracted ray.

parts of a ray diagram

Boundary line	The boundary line is a line between the two mediums.
Normal line	The normal line is a line perpendicular (at a right angle) to the boundary of two mediums. This is usually drawn as a dashed line.
Incident ray	The incident ray is a line drawn at the angle of incidence to the normal. It is directed towards the point at which the boundary and normal intersect.
Refracted ray	The refracted ray is a line drawn at the angle of refraction to the normal. It is on the other side of the boundary to the incident ray.

When going from one medium to another, the angle of a ray (relative to the normal) will change.

A good way of remembering how the angle of a ray bends when refracting is using the mnemonic FAST.

Faster Away (from the normal), Slower Towards (the normal).

This means that if the ray goes from a optically more dense medium to a optically less dense medium, it will speed up and the angle of refraction will be larger then the angle of incidence.

If the ray goes from a optically less dense medium to a optically more dense medium, it will slow down and the angle of refraction will be smaller than the angle of refraction.

Example

Draw a ray diagram for a light ray entering a optically more dense medium at $$60\degree$$​ from the normal.

Draw the boundary and the normal

Draw the incident ray - at $$60\degree$$​ from the normal

Draw the refracted ray. 

Light ray goes from less dense to more dense and will slow down. Using FAST tells you that the ray will bend towards the normal.

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