ultraviolets

Ultraviolets, or ultraviolet radiation, are electromagnetic waves with wavelengths shorter than visible light but longer than X-rays. In common classifications, UV is divided into three bands: UVA (315–400 nm), UVB (280–315 nm), and UVC (100–280 nm). The atmosphere, especially the ozone layer, absorbs most of the shorter-wavelength UV, so only UVA and part of UVB reach the Earth's surface in significant amounts.

The Sun is the primary natural source of ultraviolet radiation. Artificial sources include mercury lamps, UV LEDs, germicidal lamps, and tanning beds. UV emissions are widely used in science and industry, from curing polymers and disinfecting surfaces to fluorescence excitation and solar simulators for experiments.

UV photons carry enough energy to induce electronic transitions and chemical changes in molecules. In biology, UV exposure can damage DNA by forming thymine dimers, contributing to skin cancer and photoaging; UVB is more energetic and biologically active than UVA, while UVC is strongly absorbed by living tissues and the atmosphere. UV light also enables vitamin D synthesis in the skin.

Protection and safety are important considerations. The ozone layer provides critical shielding against the most dangerous short-wavelength UV; for people on the ground, protective measures include sunscreen, clothing, and sunglasses. Excessive exposure can cause sunburn, eye damage, and immune effects in humans, and can affect ecological systems.

Applications of ultraviolets include sterilization and disinfection of water and surfaces, photolithography in semiconductor manufacturing, fluorescence and imaging in research and forensics, plant growth regulation in agriculture, and various diagnostic and industrial processes. The study of UV radiation continues to influence environmental science, health, and technology.