Ultraviolet

The sun sends out special light. We cannot see this light. It can give us a sunburn. This light also helps us stay healthy. It helps our bodies make a special vitamin.

Some light is very strong. This strong light can hurt our skin. It can also hurt the tiny parts inside us.

Our air helps keep us safe. The air stops most of the strong light. This lets us live on land.

Some animals can see this light. Birds and bugs can see it well. They use it to see the world.

This light can also make things glow. Some rocks look very bright under it. It is a very busy kind of light.

Ultraviolet light is a type of light we cannot see. Its name means "beyond violet." This is because it has shorter waves than violet light.

Most UV light comes from the Sun. It makes up about 10% of the Sun's light. The Earth's atmosphere acts like a shield. It blocks most of the strongest UV rays. This shield allows life to live on dry land. Without it, the Sun would be too strong.

There are different kinds of UV light. Some rays are weak and reach the ground. This light helps humans make vitamin D. Other rays are very strong. They can damage DNA, which are the tiny parts inside our cells. This strong light can cause sunburns or skin cancer.

UV light can also make things glow. This is called fluorescence. Some minerals look very bright under UV light. While humans cannot see these rays, some animals can. Birds and many insects use UV light to see the world.

Ultraviolet light is a special kind of energy from the electromagnetic spectrum. Its name means "beyond violet" because it has shorter wavelengths than violet light. These wavelengths range from 100 to 400 nanometers. Some very short waves are called extreme ultraviolet. These waves are even shorter than 100 nanometers. They share some traits with X-rays. This light is not something humans can see with our eyes. However, it is all around us in the world.

This radiation works by carrying different amounts of energy. Photons, which are tiny particles of light, carry energy between 3.1 and 12 electron volts. This is enough energy to start chemical reactions. Longer UV waves can make molecules vibrate or spin faster. This action increases the temperature of the molecules. Shorter waves have even more power. They can knock electrons loose from atoms. This is called ionizing radiation. This process can break chemical bonds in organic molecules. It can even damage the DNA inside living cells.

A scientist named Johann Wilhelm Ritter discovered this light in February 1801. He was a physicist from Germany. He noticed that invisible rays made silver chloride paper turn dark very quickly. He first called these "chemical rays." Later, people used the name "ultraviolet." In 1878, researchers found that short waves could kill bacteria to clean surfaces. By 1903, scientists knew the best waves for this were around 250 nm. In 1960, experts proved how UV light affects DNA.

The Sun is our main source of this light. It makes up about 10% of the Sun's total energy output. Our atmosphere acts like a protective shield for the Earth. It blocks most of the dangerous UVC rays. The ozone layer also blocks most of the UVB rays. Most of the UV that reaches us is UVA. This light helps humans and animals make vitamin D. However, too much exposure can cause sunburns or skin cancer.

Even though we cannot see it, many animals can. Birds, insects, and some mammals can see near-ultraviolet light. Birds are special because they have a fourth color receptor. This gives them "true" UV vision to see the world. We can also see the effects of UV light through fluorescence. This is when light makes certain things glow brightly. You can see this happen with many different minerals under a special lamp.

Ultraviolet radiation, often called UV, is a form of electromagnetic radiation. It exists on a spectrum of wavelengths between 100 and 400 nanometers. These wavelengths are shorter than visible light but longer than X-rays. UV radiation is a vital part of the Sun's energy output. It makes up approximately 10% of the total electromagnetic radiation the Sun sends to Earth. Understanding UV is important because it interacts with organic molecules in complex ways. These interactions can be helpful, such as helping us make vitamins, or harmful, such as damaging living cells.

The way UV radiation works depends on the energy of its photons. Photons are tiny particles of light that carry specific amounts of energy. For ultraviolet, this energy ranges from about 3.1 to 12 electron volts. This energy level is significant because it is near the minimum required to ionize atoms. Ionization occurs when an atom loses an electron. Long-wavelength UV does not have enough energy to ionize atoms. Instead, it excites the vibrational or rotational states of molecules. This process increases the temperature of those molecules. However, short-wave UV is considered ionizing radiation. This means it has enough energy to break chemical bonds and damage DNA.

Scientists divide the ultraviolet spectrum into several distinct subtypes based on wavelength. The ISO standard 21348 identifies several specific ranges. Ultraviolet A, or UVA, has wavelengths from 315 to 400 nanometers. It is often called "soft UV" because it is not absorbed by the ozone layer. Ultraviolet B, or UVB, ranges from 280 to 315 nanometers. This is "intermediate UV" and is mostly absorbed by the ozone layer. Ultraviolet C, or UVC, ranges from 100 to 280 nanometers. This is "hard UV" and is completely absorbed by the atmosphere. There are also even shorter wavelengths called extreme ultraviolet, or EUV. These range from 10 to 121 nanometers and are highly ionizing.

The history of UV discovery began in February 1801. A German physicist named Johann Wilhelm Ritter discovered these rays. He noticed that invisible rays beyond the violet end of the spectrum darkened silver chloride-soaked paper very quickly. He originally called them "(de-)oxidizing rays" to highlight their chemical reactivity. Later, people used the term "chemical rays" throughout the 19th century. In 1878, researchers discovered that short-wavelength light could sterilize surfaces by killing bacteria. By 1903, scientists identified that the most effective wavelengths for sterilization were around 250 nm. In 1960, the specific way UV radiation affects DNA was finally established.

Earth's atmosphere plays a critical role in managing the UV that reaches the surface. Without this protection, life could not survive on dry land. The atmosphere absorbs the most energetic, short-wavelength extreme UV below 121 nm. This process ionizes the air, causing the radiation to be absorbed before it hits the ground. The ozone layer is also vital for protection. It blocks almost all UVC and most UVB radiation. Because of this, most of the UV reaching the ground is UVA. At the zenith, when the Sun is highest, the atmosphere blocks about 77% of solar UV. At ground level, sunlight is composed of 44% visible light, 3% ultraviolet, and the rest is infrared.

While UV can be dangerous, it also provides surprising benefits and interesting visual effects. For most land vertebrates, including humans, UVB is responsible for the formation of vitamin D. This is a necessary nutrient for health. UV light also causes fluorescence. This happens when radiation induces chemical reactions that make substances glow. You can see this effect in many different mineral samples. Additionally, humans cannot see UV light because our eye lenses block most of it. However, many other animals have different capabilities. Insects, birds, and some mammals can see near-ultraviolet light. Birds are particularly advanced because they have a fourth color receptor. This allows some small birds to have "true" UV vision.

Ultraviolet radiation connects to many different scientific fields and technologies. In medicine and biology, it is used for sterilization and studying DNA. In manufacturing, vacuum ultraviolet (VUV) is used in photolithography for making semiconductors. This requires operating in an oxygen-free atmosphere because oxygen absorbs VUV wavelengths. In astronomy, researchers use specialized telescopes to study extreme ultraviolet light from stars. Very hot stars, such as O-type and B-type stars, emit much more UV radiation than our Sun. Even the planets in our solar system are affected, such as the aurora at Jupiter. This shows how UV radiation is a fundamental part of how the universe works.