When a ray of light from air enters a denser medium, it :

When light travels from a rarer medium (like air) into a denser medium (like water or glass), its speed decreases. Due to this change in speed, the light ray bends towards the normal at the boundary between the two media. This phenomenon is governed by Snell's Law, which relates the angle of incidence and refraction to the refractive indices of the two media.