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"Snell's law, also known as the Snell–Descartes law, is a formula used to describe the relationship between the angles of refraction when referring to light or other waves passing through a boundary between two different isotropic media, such as water, glass and air"- Wikepedia
What this means is that the law basically describes the correlation between the angle from which light hits the surface of another medium, such as water or glass, and the way you perceive this. Most people have experienced this law firsthand when you saw a straw that is half in a glass of water and half out. The straw looks bent based on the way the light refracts through the water and glass to meet your eyes. The law originates from the fact that light travels in waves, and it is also believed that this law was derived from Fermat's principle.