Like ocean waves, light waves travel in a succession of wave fronts. When light from above a calm lake travels straight down into the water, perpendicular to the water’s surface, each wave front will continue straight toward the lake bottom. But if the light hits the water’s surface at an angle, it will bend toward the bottom of the lake when it reaches the surface.
Which of the following best explains why light bends in this way?
The theory provided in the first choice makes sense. When a wave front reaches the water surface at an angle, one end reaches the water first, and the other end reaches the water last. Water slows the speed of light, so the leading edge of the wave front slows first as the trailing end “catches up” to it. This is why light appears to bend when it enters water at an angle to the surface.