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What is UV and where does it come from?

Although very distant from our planet (~ 150 million km on average), the Sun provides us with light energy and heat, both necessary to sustain life on Earth. This energy arrives in the form of electromagnetic radiation, which is wave-like in nature and therefore is characterized by a wavelength related to the composition and temperature of the transmitter, in this case, the Sun.

The electromagnetic spectrum describes the distribution of the different types of radiation over a wide wavelength spectrum. In descending order of wavelength, the spectral range extends from radio waves (long wavelengths) up to tiny gamma rays (small wavelengths), with the infrared (IR), visible and ultraviolet (UV) ranges in between, where the Sun emits the most of its radiation.


Our human eye is only sensitive to a small fraction of the solar spectrum called the "visible" (between 4*10-7 m and 7*10-7 m) that includes all colors of the rainbow (from violet to red). Just below this range, between 1.0*10-7 m and 4*10-7 m, we find the ultraviolet rays (UV), especially energetic but invisible to the human eye; and beyond the visible and longer than 7*10-7 m, infrared wavelengths begin. This range is associated with the concept of heat as hot objects emit such radiation. At BIRA-IASB we mainly focus on UV.

Lucky we are that the atmosphere is acting as a protector shield against dangerous gamma rays, X-rays and the major part of UV because all of them are particularly harmful.

Note that theses UV rays are subdivided in three bands, depending on their energetic level:

Range Wavelength bands Hazards for life on Earth


315-400 nm

Benign effects. Nevertheless, UV-A represent 98% of global UV flux on Earth.


280-315 nm

Particularly dangerous. Fortunately UV-B represent only 2% of the overall flux on Earth.


100-280 nm

Totally absorbed by the atmosphere, UV-C does not reach the Earth’s surface.


Back to main article about UV at the surface


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