Light travels in straight lines when the medium's density is constant.

When light enters a different density perpendicularly, it also travels in a straight line.

When light enters a higher density at an angle, it bends towards an imaginary line perpendicular to its pont of entry.

When light enters a lower density medium, it bends away from the imaginary perpendicular line. This is seen with objects partially submerged in water. The light exits the water and bends away from its normal path, making the object appear crooked in relation to the unsubmerged portion.

Sometimes light will be reflected at the boundary of two mediums. This is seen in optic fibres.