What are optically denser & optically rarer mediums?

A medium ‘A’ is said to be optically denser than medium ‘B’, if the speed of light in ‘A’ is lesser than the speed of light in ‘B’ i.e the refractive index of ‘A’ is greater than the refractive index of ‘B’.

A medium ‘A’ is said to be optically rarer than the medium ‘B’, if the speed of light in ‘A’ is greater than the speed of light in ‘B’ i.e the refractive index of ‘A’ is lesser than the refractive index of ‘B’.

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When light travels from optically denser medium to an optically rarer medium, the light moves away from normal ( normal is the perpendicular drawn to the surface of the mediums at the point where the light leaves one medium and enters the other medium. The dotted line represents the normal )

Similarly when light travels from an optically rarer medium to optically denser medium, the light moves towards the normal.