This might be a silly question. Ever since I can remember, I've been taught that when light crosses the boundary between two mediums with different refractive indices, its trajectory changes due to a reduction or increase in its speed. I've never given this a second thought, but today I realized I have absolutely no clue why this happens. My problem is in making the leap from light slows down to therefore it turns. In my mind, slowing down would only mean that it gets somewhere a little later. So if it takes a beam of light 1 nanosecond to get from point A to B in a vacuum, it should take 1.5 nanoseconds if the medium was water, but that's about the only difference I'm conceptualizing as a direct consequence of light "slowing down". Just like if a car were to slow down from 100 MPH to 70 MPH; I would not expect this to cause the car to turn, say, right, by 30 degrees - it would keep moving in the same direction as before, only slower. What am I missing? Thanks.