Light

Light helps us see the world. The sun is our main source of light. People also use fire and electric lamps.

Light moves very fast. It travels in waves. Sometimes light can bend. This happens when it moves through water.

This can make a straw look bent. Light is a wonderful thing to study!

Light is a type of energy that we can see.

It is part of a big group called electromagnetic radiation. This group includes many things, like radio waves and X-rays. We call the part we can see "visible light."

Visible light has a set of wavelengths. These are the distances between the peaks of the light waves.

Light moves very fast. In a vacuum, it moves at about 299,792,458 meters per second. This is a constant speed in nature. Light can also act like tiny particles. We call these particles photons.

Light can bend when it moves between different materials. This is called refraction. For example, light bends when it goes from air into water.

This can make a straw look bent in a glass.

Most of our light comes from the Sun. Other sources include fire and electric lamps.

Some objects are opaque. This means light cannot pass through them. Other objects are transparent. This means light can go right through them.

Light is a special kind of energy that helps us see the world.

Scientists call this energy electromagnetic radiation. Most of this radiation is invisible to our eyes. We only see a small part called visible light. This part has wavelengths between 400 and 700 nanometres.

Other types of radiation sit right next to it. Infrared has longer wavelengths and is just below visible light. Ultraviolet has shorter wavelengths and sits just above it. Together, these are called optical radiation.

Light works in two different ways at once. It can act like a wave that moves through space. It can also act like tiny particles called photons.

These photons are massless, which means they have no weight. When light hits an object, how it behaves depends on the wavelength. If light passes from air into water, it changes direction. This bending is called refraction.

You can see this when a straw looks bent in a glass.

Many people have worked to understand how light moves. In the 1600s, Galileo tried to measure its speed. Later, a Danish physicist named Ole Rømer studied the moon Io. In 1676, he found light takes about 22 minutes to cross Earth's orbit.

In 1849, Hippolyte Fizeau used a spinning wheel to find the speed. He sent light to a mirror several kilometers away. Later, Albert A. Michelson used rotating mirrors to get even better results. He measured the speed between two mountains in California.

Light travels at a constant speed in a vacuum. This speed is exactly 299,792,458 meters per second. This is one of the most important numbers in nature. When light moves through things like water, it slows down. In water, light moves at about 3/4 of its vacuum speed. The Sun is our main source of natural light. It sends much of its energy to Earth as visible light.

We use light in many parts of our daily lives. We used fire for light long ago. Now, we mostly use electric lights to see at night.

We also use lenses to change how light looks. Magnifying glasses and microscopes use refraction to make things appear larger. Some objects are transparent and let light pass through them. Other objects are opaque and block the light entirely.

This is how we interact with the world every day.

Light is a form of electromagnetic radiation that can cause a visual sensation. In physics, the term "light" can describe any electromagnetic radiation, including radio waves, microwaves, X-rays, and gamma rays. However, when people speak of light in an everyday sense, they usually mean visible light. This is the specific part of the spectrum that the human eye can perceive.

Visible light is defined by its wavelengths, which generally range from 400 to 700 nanometres (nm). These wavelengths correspond to frequencies between 750 and 420 terahertz. This visible band is nestled between infrared radiation and ultraviolet radiation. Together, these three types of radiation are known as optical radiation.

To understand how light works, we must look at its dual nature. Light exhibits properties of both particles and waves. The individual, massless particles of light are called photons. These quanta of light can be detected using specialized scientific equipment. At the same time, light behaves like a wave, which allows for phenomena such as interference.

The way light interacts with matter depends on its wavelength and the energy of its photons. For example, visible light photons have just enough energy to cause electronic excitation in molecules. This excitation can lead to changes in the chemical bonding of a molecule. This process is how light triggers the sensation of vision in the human retina.

Different types of radiation interact with the world in distinct ways. Infrared radiation sits at the lower end of the visible spectrum with longer wavelengths. Because its photons lack the energy to cause lasting molecular changes in human retinal molecules, it is invisible to us. However, some animals can sense it. Snakes use a form of natural thermal imaging to detect infrared radiation. In these animals, infrared radiation raises the temperature of tiny packets of cellular water. Above the visible spectrum lies ultraviolet light, which has shorter wavelengths. Humans cannot see ultraviolet light because our cornea and internal lens absorb it. Furthermore, the rods and cones in our retina cannot detect these short wavelengths and can actually be damaged by them.

Measuring the speed of light has been a major goal for physicists throughout history. The speed of light in a vacuum is a fundamental constant of nature. It is exactly 299,792,458 meters per second. In the seventeenth century, Galileo attempted to measure this speed. In 1676, the Danish physicist Ole Rømer made a significant breakthrough. By observing the moon Io, he calculated that light takes about 22 minutes to cross the diameter of Earth's orbit. Later, in 1849, Hippolyte Fizeau used a rotating cog wheel and a distant mirror to calculate the speed more accurately. In 1862, Léon Foucault improved these methods using rotating mirrors. Finally, Albert A. Michelson conducted highly precise experiments from 1877 until 1931. He used rotating mirrors to measure light traveling between Mount Wilson and Mount San Antonio in California.

Light behaves differently depending on the medium through which it travels. While light moves at its maximum speed in a vacuum, it slows down in transparent substances. For instance, light travels at about 3/4 of its vacuum speed when moving through water. This change in speed often leads to refraction. Refraction is the bending of light rays as they pass from one transparent material into another. This happens because the wavelength of the light changes while the frequency remains constant.

This principle is essential for the function of lenses. We use refraction in magnifying glasses, spectacles, microscopes, and telescopes to manipulate how images appear. Some objects are transparent and allow light to pass through, while opaque objects reflect or absorb the light instead.

Light also comes from various sources through different physical processes. The Sun is the primary natural source of light on Earth. Solar radiation peaks in the visible region, with about 44% of the radiation reaching the ground being visible.

Other sources include thermal radiation, where a body emits light based on its temperature. As an object gets hotter, its peak emission shifts from the infrared toward the visible spectrum. This is why heated metal can glow red, then white, and eventually a blue-white color. Humans have historically used fire for light, but electric lighting has largely replaced it. Atoms can also emit light at specific energies, creating unique emission lines. This is how neon lamps and lasers function.

The study of light and its interaction with matter is known as optics. This field is divided into several specialized branches based on the scale of the observation. Geometrical optics is used to understand large-scale interactions like those involving eyes, cameras, and mirrors. Physical optics focuses on wave properties, such as diffraction and interference. Finally, quantum optics is used to study how individual photons interact with matter. These different perspectives allow scientists to understand everything from the way we see a rainbow to the complex behavior of subatomic particles.