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→Key Stage 4
: [[Wave]]s travel in a straight line until they reach an [[interface]] with a new [[medium]]. If the [[wave]] can be [[transmitted]] through the new [[medium]] it will change direction. This is called [[refraction]].
: [[Refraction]] is caused by a [[wave]] changing [[Wave Speed|speed]] as it crosses an [[interface]] from one [[medium]] into another.
: [[Refraction]] can occur gradually through a [[medium]] if the [[property|properties]] of that [[medium]] change gradually. This is the case for [[Seismic Wave|seismic waves]] travelling through the [[Earth]] as the [[material]] gets more [[dense]] as you go deeper into the [[Earth]]. This gradually increases the [[Wave Speed|speed]] of the [[wave]]s.
: When a [[wave]] '''refracts''' the [[frequency]] remains constant but the [[wavelength]] and [[Wave Speed|wave speed]] change.