Ultraviolet light is light with wavelengths too short for human eyes to see, from about 10 to 400 nanometers. A nanometer is a billionth of a meter. The explorer shows ultraviolet light with a wavelength of 60 nanometers. That wavelength lies in the extreme ultraviolet, the part of the band closest to X-rays.

Wavelength

Light travels as a wave, and its wavelength is the distance from one crest of the wave to the next. Ultraviolet light has shorter wavelengths than visible light. The shorter the wavelength, the more energy the light carries.

Scientists divide ultraviolet light into bands, and different fields draw the borders in different places. The International Commission on Illumination notes that the range from 100 to 400 nanometers is commonly divided into UV-A (315–400 nanometers), UV-B (280–315) and UV-C (100–280). ISO 21348, an international standard for measuring the Sun's light, adds bands of shorter wavelengths. One of them is the extreme ultraviolet, from 10 to 121 nanometers.

A 60-nanometer wave is extreme ultraviolet light under this definition and under narrower ones. NASA's archive for high-energy astronomy, for example, places the extreme ultraviolet at 10–90 nanometers. It notes that the borders between bands are "a matter of definition rather than science".

Size comparisons

About 1,300–1,700 waves of 60-nanometer light, laid end to end, would span the width of a human hair, which is 80–100 micrometers wide. A micrometer is a thousandth of a millimeter.
The waves of violet light, the shortest that people can see, are about 380–400 nanometers long, six to seven times as long as these.
A hepatitis B virus particle is 42 nanometers across, about 70 percent of one wavelength.
If one wavelength were stretched to 1 meter (3.3 feet), a red blood cell would be about 125 meters (410 feet) across, and a human hair would be 1.3–1.7 kilometers (0.8–1 mile) wide.

Where extreme ultraviolet light goes

The Sun gives off ultraviolet light of every wavelength. Its extreme ultraviolet light comes from the outer layers of its atmosphere, the chromosphere and the corona. None of it reaches the ground. Earth's upper atmosphere absorbs all of it more than 80 kilometers (50 miles) up.

That energy heats the thermosphere, the layer of the atmosphere between 80 and 600 kilometers (50 and 370 miles) up. It also knocks electrons off atoms and molecules there, which creates the ionosphere, a layer of electrically charged gas. Because this light never reaches the ground, it can only be measured from rockets and satellites.

Longer ultraviolet wavelengths reach deeper into the atmosphere. The atmosphere absorbs almost all UV-C light, and ozone absorbs about 95 percent of UV-B. The UV-B that does reach the ground causes sunburn.

How ultraviolet light was found

Johann Ritter found ultraviolet light in 1801. He knew that paper coated with silver chloride turned black faster in blue light than in red light. He placed paper beyond the violet end of the visible spectrum, where no light could be seen, and it turned black. Something invisible was reaching the paper.

Human eyes still cannot see ultraviolet light, but some insects can. Bumblebees, for example, can see near-ultraviolet light, the band next to violet.

Sources

Ultraviolet radiation. International Commission on Illumination, International Lighting Vocabulary, 2020.
ISO 21348:2007, Space environment: Process for determining solar irradiances (preview). International Organization for Standardization, 2007.
What are the Energy Range Definitions for the Various Types of Electromagnetic Radiation?. NASA High Energy Astrophysics Science Archive Research Center, 2021.
Solar EUV Irradiance. NOAA Space Weather Prediction Center.
Ultraviolet Waves. NASA Science, 2023.
Anatomy of an Electromagnetic Wave. NASA Science, 2023.
Visible Light. NASA Science, 2023.
Hepatitis B Virus Subviral Envelope Particle Morphogenesis and Intracellular Trafficking. Patient et al., Journal of Virology, 2007.
Just How Small Is "Nano"?. National Nanotechnology Initiative.
Erythrocytes. In Anatomy and Physiology, 2nd edition. OpenStax, 2022.

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