请英语达人帮帮忙! 求这一段话的paraphrase
问题描述：
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.
答
光的速度随物质通过是通过更慢,光时气流通过致密物质.时发生折射光的方向移动时变化在倾斜从物质(中)到另一个不同的密度(例如,在水和空气,或空气、玻璃).这是由于改变光的速度是在经过两个媒体之间的界限