Refraction

In physics, refraction is when a wave bends as it moves from one medium to another. This happens because the wave’s speed changes in a new material. You might notice refraction when you look at light through water or glass. It can also affect other waves, like sound waves and water waves.

A ray of light being refracted in a plastic block

Every material bends light a little differently. This is called the refractive index. It helps tools like optical prisms and lenses to focus or spread light. Our human eye uses refraction to bring light into focus so we can see.

Because how much light bends depends on its wavelength, different colors bend differently. This is called dispersion. It is what makes the colors of rainbows when white light passes through raindrops. Some prisms can split light into its spectral colors, showing all the colors that make up white light.

Law

For light, refraction follows Snell's law. This law explains how light changes direction when it moves from one material to another. It says that the angles at which the light hits and bends are related to how fast light travels in each material. Each material has a special number called the refractive index.

General explanation

When a wave moves into a slower medium the wavefronts get compressed. For the wavefronts to stay connected at the boundary the wave must change direction.

Refraction is when a wave, like light, changes direction as it moves from one material to another. This happens because the wave slows down in the new material. For example, when light goes from air into water, it slows down and bends. This bending happens because one part of the wave enters the new material first and slows down before the rest, causing the whole wave to change direction.

Refraction explains many everyday sights. Have you ever noticed how a straw looks bent in a glass of water? That’s refraction! It also creates rainbows and makes objects appear in different places than they really are. All these effects happen because light changes speed and direction when it passes through different materials.

On water

When light goes through water, it bends. Water has a different refractive index than air. This bending changes how things in water look. For example, a pencil placed partly in water will seem to bend at the water's surface. The water may also look shallower than it really is.

This change matters for things like spearfishing. People must aim differently because fish look like they are in a different spot than they really are. An archer fish also needs to make special adjustments when hunting from under the water.

Atmospheric

Main article: Atmospheric refraction

Light bends in the air because of the air's density. This density changes with temperature and pressure. As you go higher, pressure drops, so light bends toward the Earth. This makes stars look different when they are close to the horizon. It can also make the sun appear a little earlier at sunrise.

When temperatures in the air change, like near a hot road, light can bend in strange ways. This can make things look shimmery or move around, like when you see a heat haze. These changes can also create cool effects like mirages, where it looks like there is water on a hot road, but there isn’t any.

In eye care

In medicine, especially in optometry, ophthalmology, and orthoptics, refraction is a test that helps eye doctors see if someone has trouble seeing clearly. They use special tools like a phoropter and corrective lenses to find out which ones help a person see better. Sometimes, refractive surgery can fix common vision problems.

Mechanical waves

Water

Main article: Water wave refraction

Water waves slow down when they reach shallower water. This slowing down makes the waves change direction, which is called refraction. You can see this in special tanks called ripple tanks. It also explains why waves often reach the shore at an angle.

Sound

Sound waves can bend or change direction when they move from one place to another where the speed of sound is different. This change can happen because of differences in temperature, saltiness, or pressure of the water. The same kind of bending can occur with sound in the air around us. People have known about this for a long time, and it helps designers build highways and noise barriers to control how we hear sounds.

Law