Ultraviolet

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.