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## Reflection and Refraction

When waves strike a material boundary i.e. go from one medium to another the following may happen:

 Reflection The wave can bounce back e.g. light striking a mirror Refraction The wave can change speed and direction e.g. light travelling from air into water Absorption The wave can give up its energy e.g. microwaves are absorbed by food in a microwave oven.

## Reflection

This occurs when a wave travelling in one medium strikes the surface of a different medium and changes direction so that it returns back into the medium in which it was originally travelling in.  Simply put the waves bounce back.  Examples of reflection are light waves striking a mirror or echoes in which sound waves are reflected of a solid surface.

When waves undergo reflection the angle the incoming waves make with the surface is equal to the angle of the reflected waves. This is the Law of Reflection and can be written as:

The angle of incidence = the angle of reflection

### Reflection in use – The Periscope

A periscope is an optical instrument used predominantly in submarines so that the submarine crew can see what is happening above the surface of water without having to surface. The animation below describes how a periscope works.

Important to Remember!
• The speed of a wave is affected by the medium in which the wave is travelling through or along.
• A change in wave speed causes a change in wavelength as the frequency cannot change.
• The normal line is an imaginary construction line at right angles (perpendicular) to the refracting or reflecting surface at the point of incidence.
• When light waves travel into a denser medium the waves are refracted towards the normal and on leaving the denser medium are refracted away from the medium. This is because light travels slower in the denser medium.
• When sound waves travel into a denser medium the waves are refracted away from the normal and on leaving the denser medium are refracted towards the medium. This is because sound waves travel faster in a denser medium.