Refraction

Refraction is the bending of light as it passes from one transparent material into another (like from air into water, or from air into glass). The bending happens because light travels at different speeds in different materials. Light is fastest in a vacuum (and in air, close to vacuum speed). It slows down in glass, water and other materials. As it slows or speeds up at the boundary, its direction shifts. Refraction is what makes lenses and prisms work, why a straw in water looks broken, why ponds look shallower than they really are and how rainbows form.

• What it isLight bending in a new mediumCaused by speed change
• Speed in vacuum299,792,458 m/sThe cosmic top speed
• Speed in glassAbout 200,000 km/sRoughly 2/3 of vacuum speed
• Speed in waterAbout 226,000 km/s75% of vacuum speed
• Refractive indexSpeed ratioWater 1.33, glass 1.5, diamond 2.42
• Where usedLenses, prisms, eyes, camerasPlus rainbows and mirages

Why light bends

Imagine a car driving from a smooth road onto wet grass at an angle. The wheel that touches the grass first slows down, while the other wheel keeps going at road speed. The car briefly slews sideways, changing direction.

Light does something similar. As a light wave hits the boundary between two materials at an angle, one side of the wave enters the new material (and slows down or speeds up) before the other. The wave swings and changes direction at the boundary. Once fully inside the new material, it continues in a straight line, but in the new direction.

The bending rule

The exact bending follows a rule called Snells law, after the Dutch scientist Willebrord Snell. Two simple consequences:

• Light going from a faster medium (like air) into a slower one (like water) bends toward the normal (the line perpendicular to the surface).
• Light going from a slower medium into a faster one bends away from the normal.

Light hitting straight on (perpendicular to the surface) does not bend at all, no matter what materials are involved.

Everyday examples

• The broken straw: a straw in a glass of water looks bent at the surface, because light from the underwater part bends as it leaves the water.
• A "shallow" pond: the bottom of a pond looks closer to the surface than it really is. Light from the bottom refracts when leaving the water, fooling your eye into seeing a higher position.
• The "missing" coin trick: place a coin in an opaque cup and step back until you cannot see the coin over the rim. Have someone pour water carefully into the cup. As the water level rises, the coin appears to "lift up" into view because of refraction.
• Diamond sparkle: diamond has a very high refractive index, which makes light bend sharply as it enters and exits the cut crystal. Combined with internal reflections, diamonds bend and split light into the famous brilliant sparkle.
• Glasses and contact lenses: a curved lens refracts light to correctly focus images on your retina.
• Mirages: hot air above a road has a different density (and refractive index) from cooler air above. Light bends in the gradient and produces what looks like reflections of the sky on the road.

Fact Light travels at exactly 299,792,458 metres per second in a vacuum, but it slows dramatically inside materials. In water it travels at 75 per cent of vacuum speed. In glass, around 67 per cent. In diamond, just 41 per cent. The "slowdown factor" is called the refractive index. The bigger the index, the more the material slows light, and the more it bends.

Lenses

A lens is a transparent piece of glass or plastic shaped to bend light by refraction. The two main types are:

• Convex lens (thicker in the middle): brings light rays together, focuses them to a point. Used in magnifying glasses, cameras, telescopes, and the lenses of your eyes.
• Concave lens (thinner in the middle): spreads light rays apart. Used in glasses for nearsightedness and in some telescope eyepieces.

By choosing the right shape and material, lens designers can produce specific effects: magnify, reduce, focus a beam to a sharp point, project a clear image onto a screen. See lenses for more.

Prisms

A prism is a triangular block of glass that bends light by refraction. Each colour of light bends by a slightly different amount: violet bends the most, red the least. The result is that a prism separates white light into a rainbow spectrum of colours.

This is why a prism shows beautiful rainbows when light shines through it. And it is the same physics that makes raindrops produce rainbows in the sky.

Did you know? Total internal reflection lets fibre-optic cables carry information across continents. Light is trapped inside a thin glass fibre by total internal reflection, bouncing along the inside of the fibre over enormous distances with almost no loss. Modern undersea fibre cables carry essentially all the worlds international internet traffic this way, using nothing but light and clever refraction.

How your eye refracts light

Your eye is a beautiful natural example of refraction. Light enters through the clear cornea (which does most of the bending), then passes through the lens (which fine-tunes the focus). The combined refraction focuses the light onto the retina at the back of the eye, where photoreceptor cells convert it into nerve signals.

Your lens can change shape (becoming more curved for close objects, flatter for distant ones) to keep the image sharp at any distance. This is called accommodation. As people grow older, the lens stiffens and accommodation weakens, which is why most adults eventually need reading glasses.

Try this Half-fill a tall glass with water and put a pencil in it. Look at the pencil from the side. It appears to bend or break at the water level because the part underwater is being seen through refracted light. Now look down at the pencil from directly above the glass: it appears straight again, because looking straight down the light is not entering at an angle and there is no bending.

Deeper dive: how mirages work

On a hot day, you may have noticed shimmering "pools" of water in the distance on a road or in the desert. As you drive closer, the pools seem to retreat or vanish. This is a classic mirage, one of the most famous illusions caused by refraction.

The cause is hot air. The road surface (or desert sand) heats up under the Sun. The air immediately above it gets very hot, while air higher up is cooler. Hot air is less dense and has a lower refractive index than cool air.

Light coming from the sky travels downward through the cooler air, hits the hot layer near the ground, and bends sharply upward, eventually entering your eyes from below. Your brain interprets the incoming light as coming from a low spot ahead of you (because thats the direction the light appears to be travelling from), making it look like there is a reflective surface (like water) on the road.

This is called an inferior mirage (because the false image appears below the real object). The most familiar version is the "wet road" mirage. In deserts, the same effect can make distant cliffs look reversed and floating.

Polar regions get the opposite effect: superior mirages, where cold air sits below warmer air (e.g. over very cold sea ice). Light bends downward, making distant objects appear higher than they really are. The result can be a city or ship floating in the sky. Historians think that some sightings of "flying islands" in old Arctic exploration accounts were superior mirages.

In all cases, the explanation is refraction: light bending as it travels through air of varying density and refractive index. The fact that we see false images at all is testament to just how sensitive the bending of light can be.

For more, see lenses and rainbows.