Light

Light, also known as visible light, is a type of electromagnetic radiation that our human eye can see. It has very tiny measurements called wavelengths between 400 and 700 nanometres. This range of wavelengths is called the visible spectrum and includes all the colors we see, from red to violet.

In science, the word "light" can mean any kind of electromagnetic radiation, not just the kind we can see. This includes gamma rays, X-rays, microwaves, and radio waves. All these waves travel at the same amazing speed in empty space: 299,792,458 meters per second. This speed is called the speed of light.

The main natural source of light for Earth is the Sun. In the past, people used fire, like campfires or candles, to give them light at night. Today, most of our light comes from electric lights that we can turn on and off easily. These lights make our lives brighter and more comfortable.

Electromagnetic spectrum and visible light

Main article: Electromagnetic spectrum

Electromagnetic radiation includes many kinds of waves, such as radio waves, microwaves, infrared, the visible spectrum we see as light, ultraviolet, X-rays, and gamma rays. Visible light has wavelengths between about 400 and 700 nanometres.

Different kinds of electromagnetic radiation act in different ways based on their wavelength. Visible light can change how molecules work, which helps us see. Some animals, like some snakes, can sense infrared light using heat instead of vision.

Speed of light

Main article: Speed of light

The speed of light in a vacuum is very fast—about 186,282 miles per second. This speed is the same for all kinds of light when they travel through a vacuum.

Many physicists have worked to measure how fast light travels. One early try was made by Ole Rømer, a scientist from Denmark, in 1676. He watched Jupiter and its moon Io to guess how long light takes to travel. Later, Hippolyte Fizeau in 1849 and Léon Foucault in 1862 used mirrors and spinning wheels in their experiments. Albert A. Michelson made even better measurements in the early 1900s.

Optics

Main article: Optics

The study of light and how it interacts with matter is called optics. There are different types of optics for different uses. Geometrical optics helps us understand how eyes, lenses, cameras, fiber optics, and mirrors work. Physical optics looks at how light waves behave, especially when they bend around edges or overlap. Quantum optics studies how tiny particles of light, called photons, interact with matter.

Some objects, like glass, let light pass through them. These are called transparent objects. Other objects, like wood or metal, do not let light pass through. Instead, they reflect or absorb the light. Most objects scatter light, meaning they bounce it off in many directions. This gives them a glossy look. This scattering happens because of tiny rough spots on the surface or differences inside the material.

Refraction is when light bends as it passes from one material to another, like from air to water. This bending follows a rule called Snell’s Law. Refraction is used in tools like magnifying glasses, spectacles, contact lenses, microscopes, and telescopes to change how big or small things look.

Light sources

Further information: List of light sources

There are many sources of light. The Sun is a natural source of light. It gives off light that we can see. Other everyday sources include light bulbs, which glow when heated, and flames, which also give off light.

Atoms can emit light in specific colors. This is why neon signs glow in particular colors. Living things like fireflies produce their own light through a process called bioluminescence. Some materials glow when energy touches them, such as television screens and computer monitors. There are many other ways light can be created.

Measurement

Main articles: Photometry (optics) and Radiometry

Light can be measured in two main ways. Radiometry measures the total strength of light. Photometry measures how bright light looks to our eyes. Photometry is useful for figuring out how much light people need to see clearly.

Our eyes have special cells that help us see different colors. Because of this, two lights with the same strength might not look the same brightness. Photometry considers this, so it’s a better way to measure how bright something appears.

Light pressure

Main article: Radiation pressure

Light can push on objects it hits. This happens because tiny particles of light, called photons, bump into things and give them a little push. For most everyday objects, this push is so small we don’t notice it. But scientists are studying ways to use this push to move very tiny machines, like those used in computers.

In space, this light push could help move spacecraft using special sails that catch sunlight, kind of like a boat’s sail catching wind.

Historical theories about light, in chronological order

Classical Greece and Hellenism

In ancient times, people had many ideas about how we see. One early idea came from a thinker named Empedocles, who lived around 400 BC. He thought that our eyes could make light, like a fire, and this light helped us see. Another thinker, Euclid, studied how light travels in straight lines and how it bounces off surfaces.

Classical India

In India, different schools of thought also explored light. Some believed light was one of the basic building blocks of the world, while others saw it as tiny flashes of energy.

Descartes

Later, a scientist named René Descartes suggested that light was a mechanical property, like a wave. He thought light moved faster through thick materials, though we now know this isn't true.

Particle theory

Some scientists thought light was made of tiny particles. Isaac Newton, a famous scientist, believed light was made of small parts called corpuscles. This idea was popular for a long time.

Wave theory

Other scientists thought light behaved like waves. Robert Hooke and Christiaan Huygens were among those who studied light as waves. They thought light moved in waves through a special substance called the luminiferous aether.

Electromagnetic theory

In the 1800s, scientists discovered that light was related to electricity and magnetism. James Clerk Maxwell showed that light was a type of electromagnetic wave. This helped explain many things about light.

Quantum theory

In the early 1900s, scientists found that light also acted like tiny particles called photons in some situations. This idea helped explain more about how light behaves, leading to the development of quantum mechanics.

Main article: Corpuscular theory of light

Main article: Electromagnetic radiation

Main article: Quantum optics

Use for light on Earth

Sunlight gives energy to green plants. These plants use that energy to make sugars. When animals eat these plants, they get the energy they need to live. This process is called photosynthesis. Some animals, like fireflies, can make their own light. Fireflies use light to find friends. Vampire squid use light to stay safe from danger.