1. UV Light

2. Ultraviolet Light
Ultraviolet (UV) light falls in the range of the EM spectrum between visible light and X-rays
UV light, which comes from the sun, is invisible to the human eye.
It makes black-light posters glow, and is responsible for summer tans — and sunburns.
However, too much exposure to UV radiation is damaging to living tissue. 

3. Ultraviolet Light
It has frequencies of about 8 × 1014 to 3 × 1016  hertz (Hz), and wavelengths of about 380 nanometers to about 10 nm.

4. Ultraviolet Light UV is generally divided into three sub-bands:
UVA, or near UV (315–400 nm);
UVB, or middle UV (280–315 nm)
UVC, or far UV (180–280 nm)

5. Ultraviolet Light
Radiations with wavelengths from 10 nm to 180 nm are sometimes referred to as vacuum or extreme UV.
These wavelengths are blocked by air, and they only propagate in a vacuum.

6. UV Ionization UV radiation has enough energy to break chemical bonds.
Due to their higher energies, UV photons can cause ionization, a process in which electrons break away from atoms.
The resulting vacancy affects the chemical properties of the atoms and causes them to form or break chemical bonds that they otherwise would not.

7. UV Ionization
This can be useful for chemical processing, or it can be damaging to materials and living tissues.
This damage can be beneficial, for instance, in disinfecting surfaces, but it can also be harmful, particularly to skin and eyes, which are most adversely affected by higher-energy UVB and UVC radiation. 

8. UV Effects
Most of the natural UV light people encounter comes from the sun.
However, only about 10 percent of sunlight is UV, and only about one-third of this penetrates the atmosphere to reach the ground.

9. UV Effects
Of the solar UV energy that reaches the equator, 95 percent is UVA and 5 percent is UVB.
No measurable UVC from solar radiation reaches the Earth's surface, because ozone, molecular oxygen and water vapor in the upper atmosphere completely absorb the shortest UV wavelengths.
Still, "broad-spectrum ultraviolet radiation [UVA and UVB] is the strongest and most damaging to living things

10. Sunburn A suntan is a reaction to exposure to harmful UVB rays.
Essentially, a suntan results from the body's natural defense mechanism kicking in. 
This consists of a pigment called melanin, which is produced by cells in the skin called melanocytes.

11. Sunburn Melanin absorbs UV light and dissipates it as heat.
When the body senses sun damage, it sends melanin into surrounding cells and tries to protect them from sustaining more damage.
The pigment causes the skin to darken. 

12. Sunburn
However, continued exposure to UV radiation can overwhelm the body's defenses.
When this happens, a toxic reaction occurs, resulting in sunburn.
UV rays can damage the DNA in the body's cells.
The body senses this destruction and floods the area with blood to help with the healing process.
Painful inflammation occurs as well. Usually within half a day of overindulging in the sun, the characteristic red-lobster look of a sunburn begins to make itself known, and felt.

13. Sunburn
Sometimes the cells with DNA mutated by the sun's rays turn into problem cells that don't die but keep proliferating as cancers.
The UV light causes random damages in the DNA and DNA repair process such that cells acquire the ability to avoid dying.

14. Sunburn
The result is skin cancer, the most common form of cancer in the United States.
People who get sunburned repeatedly are at much higher risk.
The risk for the deadliest form of skin cancer, called melanoma, doubles for someone who has received five or more sunburns.

15. Other Sources of UV Artificial sources include: tanning booths
black lights
curing lamps
germicidal lamps
mercury vapor lamps
halogen lights

16. Fluorescence
Many substances — including minerals, plants, fungi and microbes, as well as organic and inorganic chemicals — can absorb UV radiation.
Absorption causes electrons in the material to jump to a higher energy level.
These electrons can then return to a lower energy level in a series of smaller steps, emitting a portion of their absorbed energy as visible light.

17. Fluorescence
Materials used as pigments in paint or dye that exhibit such fluorescence appear brighter under sunlight because they absorb invisible UV light and re-emit it at visible wavelengths.
For this reason they are commonly used for signs, safety vests and other applications in which high visibility is important. 

18. Fluorescence
In fluorescent tubes used for lighting, ultraviolet radiation with a wavelength of 254 nm is produced along with the blue light that is emitted when an electric current is passed through mercury vapor.
This ultraviolet radiation is invisible but contains more energy than the visible light emitted.

19. Fluorescence
The energy from the ultraviolet light is absorbed by the fluorescent coating inside the fluorescent lamp and re-emitted as visible light.
Similar tubes without the same fluorescent coating emit UV light that can be used to disinfect surfaces, since the ionizing effects of UV radiation can kill most bacteria. 

20. Fluorescence
Black-light tubes typically use mercury vapor to produce long-wave UVA light, which causes certain dyes and pigments to fluoresce.
The glass tube is coated with a dark-purple filter material to block most of the visible light, making the fluorescent glow appear more pronounced.
This filtering is not needed for applications such as disinfecting.