What Is Light?

Light is a form of energy that travels through space in straight lines. It is what lets us see, lets plants grow, warms our skin in sunshine and powers solar cells. Light is also one of the strangest things in physics: it acts both like a wave (with wavelength and frequency) and like a stream of tiny particles called photons. Light is the fastest thing in the universe, travelling at almost 300,000 km per second. Without it, the world as we know it could not exist.

What it isElectromagnetic energyActs like wave and particle
Speed299,792,458 m/sIn a vacuum
From the Sun8 minutes 20 secondsTo reach Earth
Visible rangeAround 400-700 nmColours of the rainbow
PhotonA particle of lightCarries a tiny bundle of energy
SourcesSun, fire, lightbulbs, LEDsPlus screens and lasers

Where light comes from

Light is produced by any object that has enough energy to give off photons:

The Sun and other stars: nuclear fusion in their cores produces enormous amounts of light.
Fire: hot gases and tiny soot particles glow as they burn.
Incandescent bulbs: electricity heats a thin wire filament until it glows white-hot.
Fluorescent and LED lights: convert electricity directly into light with less waste.
Lasers: produce focused beams of single-coloured light.
Bioluminescence: some animals (fireflies, deep-sea fish, certain jellyfish) produce light through chemical reactions inside their bodies.
Lightning: hot plasma briefly glows brilliantly.

Most of the things you see do not produce light themselves. They reflect light from other sources. The Moon, your books, your hands and the walls of your room are all visible only because they reflect light from the Sun or from lights in the room.

How fast?

Light travels at 299,792,458 metres per second in a vacuum. This is so fast that:

Light from a torch reaches across a room in about 30 billionths of a second.
Light from the Sun takes 8 minutes 20 seconds to reach Earth.
Light from the next nearest star (Proxima Centauri) takes 4.2 years.
Light from the centre of our galaxy takes 26,000 years.
Light from the most distant galaxies we can see has been travelling for over 13 billion years.

According to Einsteins theory of relativity, nothing can travel faster than light. The speed of light is the cosmic speed limit, the same for every observer in the universe.

Fact When you look at the night sky, you are looking back in time. Light from distant stars and galaxies left them many years ago. The North Star (Polaris) is so far away that you see it as it looked over 400 years ago, around when Shakespeare was writing his plays. Some galaxies you can see through a telescope showed themselves as they were before the Earth even formed.

Wave or particle?

One of the strangest discoveries of modern physics is that light behaves like both a wave and a particle, depending on how you look at it.

As a wave: light has wavelength, frequency, and can interfere with itself, producing bright and dark patterns. This explains how rainbows form and how a prism splits white light into colours.
As a particle: light is made of tiny "lumps" of energy called photons. Each photon carries a fixed amount of energy. This explains why photoelectric cells (in solar panels) produce electricity in steps.

This double nature is called wave-particle duality and is at the heart of quantum mechanics. Einstein won the Nobel Prize in 1921 for proving the particle nature of light.

Colours and the visible spectrum

What we call "visible light" is just a tiny range of wavelengths, from about 400 to 700 nanometres. Each wavelength looks like a different colour:

Red: longest visible wavelength, about 620-700 nm.
Orange: about 590-620 nm.
Yellow: about 570-590 nm.
Green: about 495-570 nm.
Blue: about 450-495 nm.
Violet: shortest visible wavelength, about 380-450 nm.

White light (like sunlight) is a mix of all these colours. A prism (or raindrops in a rainbow) separates them out by bending each colour by a slightly different amount. See the visible spectrum.

Did you know? Visible light is only a tiny piece of the full electromagnetic spectrum. Beyond red is infrared (used in remote controls), then microwaves (cookers and Wi-Fi), then radio waves (broadcasting). Beyond violet is ultraviolet (sunburn), then X-rays (medical scans), then gamma rays (some cancer treatments). All are the same kind of wave, just at different wavelengths. Visible light is the small fraction our eyes happen to detect.

How we see

Light enters your eye through the pupil (the dark spot in the middle), is focused by the cornea and lens, and forms an upside-down image on the back of your eye, on the retina. The retina contains millions of light-sensitive cells: rods for low-light vision and cones for colour vision (red-, green- and blue-sensitive).

The cells convert light into nerve signals that travel through the optic nerve to your brain. The brain processes the signals, flips the image right-way up and turns it into the picture you see. All this happens in a tiny fraction of a second, continuously, every waking moment.

What light does

Reflects off surfaces: enabling us to see things.
Refracts when entering a new medium: enabling lenses, prisms and rainbows.
Absorbs into materials: turning into heat or other energy.
Transmits through clear materials: glass, water, plastics.
Diffracts around obstacles: spreading out.
Polarises: vibrating in just one direction (used in sunglasses).
Drives chemistry: powers photosynthesis in plants and chemical reactions in your eyes.

Try this Stand in front of a mirror and shine a torch (carefully) at an angle. Watch how the beam reflects off the mirror at the same angle (a perfect demonstration of the law of reflection). Now shine the torch into a glass of water at an angle: the beam appears to bend at the surface. That bending is refraction. Two of the most important light behaviours, demonstrated in five minutes.

Deeper dive: how Einstein worked out the speed of light is constant

One of the most surprising facts in modern physics is that the speed of light is the same for everyone, no matter how they are moving. This is called the postulate of light, and it leads directly to Einsteins theory of special relativity.

It is strange. Imagine you are running at 10 km/h and you throw a tennis ball forward at 30 km/h. To a stationary observer, the ball moves at 40 km/h (your speed plus the balls speed). All our everyday experience tells us speeds add up like that.

Light does not work that way. If you turn on a torch while standing still, the photons fly out at 300,000 km/s. If you turn on the same torch while sitting in a fast train, the photons still fly out at 300,000 km/s relative to YOU. But (and this is the strange part) someone standing still on the platform also measures the photons going at 300,000 km/s, NOT 300,000 + the trains speed.

How can this be? In 1905, Einstein showed that if everyone measures the same speed of light, then time itself must slow down for fast-moving observers, and lengths must squash, and masses must increase. This was the birth of special relativity.

The predictions sound impossible, but they have been confirmed by hundreds of experiments. Atomic clocks flown on fast aircraft tick measurably slower than identical clocks on the ground. Subatomic particles in accelerators live longer than they should because of relativistic time dilation. GPS satellites have to correct for relativity to give accurate locations. Einsteins ideas about light have rebuilt our entire understanding of space and time.

For more, see the speed of light and the visible spectrum.