Lens

Example:

Imagine you're trying to get a marble to roll straight down a bumpy, uneven path. Just like the path changes the marble's direction, a lens redirects light because of its curved surface. In both cases, the unevenness of the path or the curvature of the lens changes the trajectory: the marble gets nudged in different directions by the bumps, while the lens bends the light as it passes through, focusing or spreading it out depending on the curve.