When light refracts, it gives the optical illusion of bending. A classic example is to place a pencil in a half full glass of water; the pencil will appear to be broken because the light travels at one speed in the medium of air, then travels at a reduced, slower speed in the medium of water.
In Newton's corpuscular theory, he stated that light was composed of tiny particles called "corpuscles", and that they traveled in direct, straight paths with constant velocities. He also said they could operate without the aid of medium, such as air and water. Although it could be argued Newton's theory makes sense in that it slows down the tiny particles as they enter the differing medium, I prefer the other theory, the wave theory. This theory says light is composed of waves that travel at a constant velocity and interact with each other. I think the idea of waves being able to travel faster in air, then slower in water, producing a visual deviation between the two, is a more plausible defense.