Final answer:
Bending of light when it passes from air to a liquid is caused by a change in speed, known as refraction. This depends on the mediums' index of refraction, with light slowing down as it enters a denser medium and speeding up when it returns to a less dense medium.
Step-by-step explanation:
When light passes from one medium (air) to another (liquid), it undergoes a change in direction, an effect known as refraction. This occurs because there is a change in light's speed as it moves between materials of different densities.
For instance, light slows down when it enters water from air because the speed of light in water is three-fourths that of its speed in a vacuum (denoted 'c'). Conversely, light speeds up when it moves from water to air. This bending of light is governed by the medium's index of refraction, and the path of light is exactly reversible.
A good analogy to understand this concept is when a lawn mower moves from a footpath to grass, the wheels slow down and the mower changes direction, similar to how light bends when it enters a slower medium.