Reflection & 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.|
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:
Examples 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.
The speed at which a wave travels is dependant upon the medium in which it travels along or through. The speed of a wave changes when a wave moves from one medium to another. This change in wave speed is accompanied by a change in wavelength and change in direction. It is this change of direction or bending of the wave as it passes from one medium to another that is called refraction.
Examples of Refraction
Effects of Refraction
Refraction is why water appears shallower than it really is. In the diagram below the rays of light from the fish are refracted away from the normal as the leave the water. This bending of the light is not registered by the eye rather it seems as if the rays come from an imaginary fish higher in the fish bowl. Therefore the apparent position of the fish is higher than the real position.
- 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.