Light

Light, also known as visible light or visible radiation, is a type of electromagnetic radiation that our human eye can see. It has wavelengths between 400 and 700 nanometres, which are very tiny measurements! 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, which have different wavelengths and uses. No matter the type, all these waves travel at the same amazing speed in empty space: 299,792,458 meters per second, a number known as 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, making our lives brighter and more comfortable.

Electromagnetic spectrum and visible light

Main article: Electromagnetic spectrum

Electromagnetic radiation includes many types 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 types of electromagnetic radiation behave in various ways depending on their wavelength. Visible light can affect molecules and change how they work, which helps us see. Some animals, like certain snakes, can sense infrared light without the same kind of vision we have, using heat to detect it.

Speed of light

Main article: Speed of light

The speed of light in vacuum is exactly 299,792,458 meters per second, which is about 186,282 miles per second. This speed is the same for all types of electromagnetic radiation, including visible light, when they travel through a vacuum.

Many physicists have tried to measure the speed of light over the years. One early attempt was made by Ole Rømer, a Danish scientist, in 1676. He used observations of Jupiter and its moon Io to estimate how long light takes to travel. Later, Hippolyte Fizeau in 1849 and Léon Foucault in 1862 used clever experiments with mirrors and rotating wheels to measure the speed more accurately. Albert A. Michelson made even more precise 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 situations. 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 individual particles of light, called photons, interact with matter.

Some objects, like glass, let light pass through them, which means they are transparent. Other objects, like wood or metal, do not let light pass through and instead reflect or absorb the light. Most objects scatter light, meaning they bounce it off in many directions, which gives them a glossy appearance. 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, emitting radiation that includes a lot of visible light. Other everyday sources include incandescent light bulbs, which produce light by heating a filament until it glows, and flames, which also give off visible light.

Atoms can emit light at specific colors, which is why neon signs glow in particular hues. Living things like fireflies also produce their own light through a process called bioluminescence. Some materials glow when exposed to energy, such as television screens and computer monitors. There are many other interesting ways light can be created, including through electrical processes and chemical reactions.

Measurement

Main articles: Photometry (optics) and Radiometry

Light can be measured in two main ways. Radiometry measures the total power of light, while photometry measures how bright light appears to the human eye. Photometry is especially useful for measuring illumination, or how much light is needed for people to see well.

Our eyes have special cells called cone cells that respond to different colors of light. Because of this, two lights with the same power might not look equally bright. Photometry takes this into account, making it a better way to measure how bright something looks compared to just measuring raw power.

Light pressure

Main article: Radiation pressure

Light can actually 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 provides the energy that green plants use to create sugars, which release energy for living things that eat them. This process, called photosynthesis, provides almost all the energy used by living things. Some animals, like fireflies, can make their own light. Fireflies use light to find mates, and vampire squid use it to hide from predators.