Definition
Refraction is the bending of light's path when it crosses the boundary between two different transparent materials, like air and water, at an angle. It happens because light travels at different speeds in different materials.
It Turns Like Wheels Entering Sand
Picture a toy car rolling straight along smooth pavement. Now say it enters a patch of soft sand at a slant. The wheel that touches the sand first slows down, but the wheel still on the pavement keeps rolling fast.
Because the two wheels have different speeds, the car's nose swings sharply toward the slower, sandy side. Light does the same thing. Light travels as a wave. When it meets a new material at an angle, the part that reaches the boundary first changes speed first, and the direction of travel bends.
How fast light moves depends on the electromagnetic properties of a material. In a vacuum or in air, it moves freely and fastest. But when it enters a material packed with atoms, like water or glass, it slows down noticeably. It's this difference in speed that causes the bend.
The Secret of the Bent Chopstick in Water
Pour water into a clear glass and stick in a straw or a chopstick. It looks as if it snapped in the middle and the two halves don't line up. Light reflected from the tip of the chopstick under the water bends toward the water's surface as it leaves the water and enters the air.
Our eyes and brain assume that light always travels in a straight line. So they trace the bent light backward in a straight line and decide the object is there. As a result, an object under water looks closer to the surface, as if lifted up, compared with its real depth.
This bending is also why the bottom of a clear stream looks shallower than you expected when you dip your foot in. And it's why, when you try to catch a fish from outside the water, you have to aim a little below where it appears.
To Be More Precise: The Shortest-Time Rule
The number that tells how much a material slows light down is called the refractive index. Taking a vacuum as 1, air is about 1.0003, water is about 1.33, glass is about 1.5, and diamond reaches about 2.4. For example, a refractive index of 1.33 for water means light moves 1.33 times slower in water than in a vacuum, or at about 75% of its vacuum speed. The bigger a material's refractive index, the more it slows light, and the more the light bends.
Physics explains this with a basic principle called Fermat's principle. Imagine a lifeguard running to save someone in the sea. If the lifeguard runs on sand much faster than they swim, swimming straight at the person isn't best. Running a bit farther along the sand and then jumping in can cut the rescue time a lot.
Light also picks, all on its own, the path that takes the least time between two points. It runs a longer distance in fast air and a shorter one in slow water, so it changes direction at the boundary. Eyeglass lenses, cameras, and the rainbow that appears after rain all exist thanks to this elegant rule of light.
Common misconceptions
Light bends because a material pushes or pulls on it with a force.
No special physical force acts on the light. When light passes through different materials, its speed changes, and the direction of the wavefront bends all by itself.
Where you meet it
A straw in a glass of water looks bent, or thicker, at the water's surface.
Eyeglass and camera lenses gather or spread light to form a sharp image.
On a hot summer day, a mirage makes it look like there's a puddle on the heated asphalt road.
In one sentence
When light enters a different material, its speed changes, and it bends to keep its travel time as short as possible.