Why does light bend when it goes from air to water? What's refractive index?

Light bends, or refracts, when it passes from one transparent medium to another because its speed changes. Light travels at different speeds in different media; it slows down when entering a denser medium like water from a rarer medium like air. This change in speed causes the light wave to change direction, unless it enters perpendicularly. The refractive index (n) of a medium is a measure of how much the speed of light is reduced in that medium compared to its speed in a vacuum. It's defined as the ratio of the speed of light in vacuum ($c$) to the speed of light in the medium ($v$), i.e., $n = \frac{c}{v}$. A higher refractive index means light travels slower in that medium and bends more when entering or leaving it. For two media, it's the ratio of speeds in those media.