General explanation thumb|When a wave moves into a slower medium the wavefronts get compressed. For the wavefronts to stay connected at the boundary the wave must change direction.
genErAL EXpLanaTIon thUMB|WhEN A wAve moves INto A slOwer MediUM the wavEfronTS gEt compreSSed. for the WavefroNtS tO sTaY conneCtEd At tHe bounDary the waVe must chANGE dirEction.