The behavior when waves slow down and change direction is called refraction. Refraction occurs when a wave passes from one medium into another with different properties, causing its speed to change. As a result, the wave bends or changes its direction. This is commonly observed with light waves, water waves, and other types of waves. When waves slow down, such as water waves moving from deeper to shallower water, they bend towards the normal line (an imaginary line perpendicular to the boundary). This change in direction due to speed change is precisely what defines refraction.