What is refraction of light?
Quick answer
Refraction is the bending of light as it passes from one material into another, such as from air into water or glass. It happens because light travels at different speeds in different materials: it slows down in water and glass, and that change in speed makes it change direction. Refraction makes a straw in a glass look bent.
Why light bends
Light travels about 300,000 km/s in a vacuum, but only about 225,000 km/s in water and 200,000 km/s in glass. Picture a shopping cart rolling from smooth pavement onto grass at an angle: the wheel that hits the grass first slows down, and the cart turns. Light behaves similarly when it enters a slower material at an angle.
The refractive index
Each material has a refractive index (n), showing how much it slows light.
| Material | Refractive index |
|---|---|
| Vacuum / air | 1.00 |
| Water | 1.33 |
| Glass | About 1.5 |
| Diamond | 2.42 |
Diamond’s high index bends light strongly and splits it into colors, giving it its famous sparkle. The exact relationship between angles and indexes is called Snell’s law.
Refraction in everyday life
| Example | What’s happening |
|---|---|
| Bent straw | Light from the underwater part bends as it leaves the water |
| Shallow-looking pools | Water looks less deep than it really is |
| Rainbows | Raindrops refract and split sunlight into colors |
| Glasses and contact lenses | Curved lenses bend light to focus it on the retina |
| Mirages | Hot air near a road bends light from the sky, looking like water |
| Twinkling stars | Moving layers of air refract starlight |
Refraction vs reflection
| Refraction | Reflection | |
|---|---|---|
| What happens | Light passes through and bends | Light bounces off |
| Example | Lens, prism | Mirror; see why mirrors reverse text |
Your eyes depend on it
Your eye’s cornea and lens refract light to form an image on the back of your eye. The colored part that controls how much light enters is explained in which part of the eye controls light. For why the sky looks blue, a different effect called scattering, see why the sky is blue.
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