Refraction
Refraction is the bending of light when it moves from one material into another, like from air into glass.
A straw in a glass of water looks bent or broken at the water line. Light travels fast through air, then slows down when it hits the water. The slowdown makes the light beam turn at an angle. That angle shifts where your eye thinks the straw ends up.
Explaining refraction by grade level
Look through a pair of glasses. Things look bigger or smaller because the glass bends light. Light travels in straight lines through air. But when it hits glass, it changes direction. That bend is what makes glasses help people see clearly. Without that bend, the world looks blurry.
Projects that explore refraction
Light bends when it moves from air into glass, and eyeglass lenses use that bending to fix vision problems. Hold a nearsighted lens at arm's length and the view looks smaller. Hold a farsighted lens the same way and it looks bigger, or even upside down. Each lens bends light a different way to fix a different problem.
Light changes speed when it passes from one material into another, and that speed change bends its path. A toy car on a cardboard ramp makes this visible. Spread glue on one half of the cardboard and cover it with salt or sugar, then roll the car at an angle across the boundary. On clean cardboard, the car rolls straight. On the gritty side it slows down and its path curves. That change in speed causing a change in direction is the same principle that makes lenses work.