So far, we know that light travels in straight lines. However, light can be bent if it changes mediums. A medium is simply a substance; for example, water is one medium, air is another. A vacuum is a space devoid of matter. So, if light is travelling through air and it suddenly travels through water, the ray of light will be bent:
The index of refraction (known as n) is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in a given material (v). Click here for further explanation on how to calculate the index of refraction for a medium.