Refraction

In physics, refraction is the bending of a wave when it moves from one medium to another. This happens because the wave’s speed changes as it enters a new material. While many people notice refraction when looking at light passing through water or glass, it also affects other types of waves, like sound waves and water waves.

A ray of light being refracted in a plastic block

Every material bends light by a slightly different amount, which is described by its refractive index. This property helps tools like optical prisms and lenses focus or spread light. Our own human eye relies on refraction to bring light into focus on the retina, allowing us to see.

Because the amount of bending depends on the wavelength of light, different colors bend by different amounts. This effect, called dispersion, is what creates the beautiful colors of rainbows when white light passes through raindrops. Certain prisms can also split light into its spectral colors, showing us all the colors that make up what we normally see as white.

Law

For light, refraction follows Snell's law. This law tells us how light changes direction when it moves from one material to another. It says that the ratio of the angles at which the light hits the new material and bends through it is related to how fast light travels in each material, or to a special number called the refractive index for each material.

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 occurs 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 travels through water, it bends because water has a different refractive index than air. This bending makes objects in water look different from what they really are. For example, a pencil placed partly in water will seem to bend at the water's surface, and the water itself may look shallower than it actually is.

This change in how we see depth is important for activities like spearfishing, where aiming must be adjusted because fish appear in a different place than they really are. Similarly, an archer fish must make special corrections when hunting from below the water's surface.

Atmospheric

Main article: Atmospheric refraction

The way light bends in the air changes with the air's density, which depends on temperature and pressure. Because pressure drops as you go higher, light bends toward the Earth when traveling long distances, making stars look slightly different when they are near the horizon. It also makes the sun appear a little earlier at sunrise than it actually is.

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

In eye care

In medicine, especially in optometry, ophthalmology, and orthoptics, refraction is a test used by eye care professionals to find out if someone has a refractive error. They use special tools like a phoropter and different corrective lenses to see which ones help a person see the best. Sometimes, refractive surgery is done to fix common vision problems.

Mechanical waves

Water

Main article: Water wave refraction

Water waves are almost parallel to the beach when they hit it because they gradually refract towards land as the water gets shallower.

Water waves move more slowly when they are in shallower water. This slowing down causes the waves to change direction, which is known as refraction. You can see this in special tanks called ripple tanks, and it also explains why waves often hit the shore at an angle close to straight on.

Sound

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

Law