Why does light behave according to the left picture and not according to the right?

Question

Light changes its speed, because the new medium is either more, or less dense; but why does it change direction?

Answer 1

Think of continuous wave fronts perpendicular to the direction of travel. At the left of the left image the wave front at a point is slowed down while the wave front at a point to the right of the left image is not slowed. As the wave front penetrates the refractor it is therefore distorted so that the front which was originally a horizontal line moving down becomes a diagonal line moving to the left.