请英语达人帮帮忙! 求这一段话的paraphrase
The speed at which light travels varies depending on the substances through which it is passing; light travels more slowly when it passes through a denser substance. Refraction occurs when the direction in which light is moving changes as it passes at an oblique angle from on substance(medium) to another of a different density (for instance, between water and air, or air and glass). This is caused by a change in the speed of light as it passes across the boundary between the two media.