What do you see when you place a pencil in a glass of water?
The pencil appears to bend where it enters the water. The pencil is not really bent. The light we see has bent as it moves from water to air. This is called refraction. This occurs because light moves more slowly in water than in air.
You can see how this happens by looking at lines of marchers with their arms intertwined moving from pavement onto sand. The marchers move faster on the pavement than in the sand. If the marchers in a given line all move from the pavement to the sand at the same time, nothing happens to their direction of movement. However, they slow down, causing the lines of marchers to be spaced closer together. Their direction of travel does not change.