When a ray of light enters a glass lens surrounded by air, it slows down.
(a) As it leaves the glass, does its speed increase, decrease, or stay the same?
(b) Choose the best explanation from among the following:
I. Its speed increases because the ray is now propagating in a medium with a smaller index of refraction.
II. The speed decreases because the speed of light decreases whenever light moves from one medium to another.
III. The speed will stay the same because the speed of light is a universal constant.