✔ 最佳答案
The following diagrams (click the links to see the diagrams) give an illustration how light is bent on crossing the air-water surface. It is because of such refraction of light (光的折射) that makes the depth of water to become shallower.
http://cnx.org/content/m40065/1.1/PG10C6_022.png
http://physics.dindon.mn/Physics/upload/image/apparent%20depth.jpg