Refraction
Definition
Light bending as it passes from one material into another — why a straw looks broken in a glass of water.
Technical definition
The change in direction of a wave crossing a boundary between media with different wave speeds, governed by Snell's law: n₁sinθ₁ = n₂sinθ₂. Frequency is unchanged; wavelength changes with speed.
Example
A straw in water looks kinked at the surface because light from the submerged part bends as it exits into air — your brain traces the rays back in straight lines and misplaces the straw's true position.
Refraction bends light because speed changes, not because the light 'hits something.' Light slows from 300,000 km/s in air to about 225,000 km/s in water, and the wavefront pivots — like a marching band wheeling when one side hits mud. No speed change, no bending: that is why light entering glass head-on (0° incidence) does not bend at all.