Refraction occurs when light travels from one medium to another which changes the speed at which the light travels. This causes light to bend upon incidence with the interface of a new material. Angle of refraction is smaller than angle of incidence when light ray travels from rarer medium to denser medium. I.e light can either refract towards the normal(when slowing down while crossing the boundary) or away from the normal (when speeding up while crossing the boundary). When light undergoes refraction, its frequency remains the same.