How can wavefronts be used to explain why waves change direction when passing from one medium to another.

Light waves are about to move from air into glass. When the first wavefronts move into the glass they slow down and become closer together, making the wavelength smaller. This causes the waves to change direction towards the normal, so they refract. Then when the waves speed up, they change direction away from the normal, while spreading apart making the wavelength larger. While when wavefronts are at right angles to the normal the waves slow down as they enter the glass, but the whole wavefront slows down at the same time. The waves, therefore, do not change direction.