Ultraviolet (UV)

These are electromagnetic waves with frequency just higher than blue light, and this band spans from $8 \times 10^{14}\, {\rm Hz}$ to $3.4 \times 10^{16} \, Hz$. Typical energy range corresponds to 3$-$100 eV.

One of the main sources of UV radiation is Sun. UV radiation has sufficient energy to ionize the atoms in the upper atmosphere, producing the ionosphere. Exposure to UV radiation is harmful to living organisms, and it is used to kill bacteria in many water purification systems. Wavelenghs less than 300 nm are well suited for this killing. Fortunately Ozone ($O_3$) in the atmosphere absorbs UV radiation protecting life on the earth's surface.

The human eyes cannot see in UV as it is absorbed by the lens. People whose lens have been removed due to cataract can see in UV to some extant. Many insects can see in part of the UV band.

UV is produced through more energetic electronic transitions in atoms. For example the transition of the hydrogen atom from the first excited state to the ground slate (Lyman-$\alpha$) produces UV.

UV is used in lithography for making ICS, erasing EPROMS, etc.