Snell’s Law explains that light bends when it travels from one medium to another because its speed changes due to the difference in optical density between the two media. When light enters a denser medium (like air to water), it slows down and bends toward the normal — the imaginary line perpendicular to the surface at the point of contact. When light moves into a less dense medium (like water to air), it speeds up and bends away from the normal.
The refractive index of each medium determines how much the light bends. A higher refractive index means the medium slows light down more. Snell’s Law shows that the greater the difference in refractive indices, the more the light will bend. This bending effect is what causes visual distortions when you look at objects underwater or through glass. It’s also why a straw in a glass of water appears bent at the surface — light rays from the straw bend at the air-water boundary before reaching your eyes.