Ultraviolet light is close in the spectrum to visible light, just past violet (from ROYGBIV - red, orange, yellow, green, blue, indigo, and violet). It is so close that although humans cannot see it, other species can.
Frequency and Wavelength
Ultraviolet is also more energetic than visible light, and is therefore more dangerous. The higher the frequency, the more energetic. A shorter wavelength corresponds to a higher frequency. All light travels with the same speed (in a vacuum). Very energetic light (such as X-rays) travels at the same speed as not so energetic light (such as radio waves). That extra energy makes the light oscillate faster (go side to side essentially) while it is traveling forward. It is the oscillation that gives the wave shape. Faster oscillations mean a more scrunched wave shape, and therefore shorter wavelengths. Frequency and wavelengths are interrelated because light has a constant speed.
Energy and Skin Damage
The more energetic a ray of light is, the more that light will blast apart the bonds between molecules in skin, damaging cells. These cells have to repair, as they do with a sunburn. The redness of the sunburn is caused by a flush of healing blood close to the surface of the skin through tiny capillaries. Worse though, are energetic light rays that penetrate the nuclei of skin cells, damaging DNA, and potentially causing skin cancer (there are many types of skin cancer).
UVA, UVB, UVC
There are three types of ultraviolet light in reference to skin protection. These are UVA, UVB, and UVC. Of these, UVC is the most damaging, it is the most energetic of the three types. It is also the least common, since it is totally blocked out by the Earth's atmosphere. UVC is present in industrial uses, such as food preparation and sanitation. Adequate protection should be implemented to guard against these sources.
UVB is the wavelength that causes most sun damage on skin. It is blocked completely by glass (not plexi-glass), though other materials can be coated to block UVB rays.
Most sun born ultraviolet is UVA, the longest and least damaging wavelength, though it still does damage. Glass is transparent to UVA rays, so it offers little protection.
Ultraviolet Protection
For best skin protection, look for products that are made to resist both UVA and UVB rays. These products will be clearly marked as such. Be aware that "UV protection" is not the same thing as "UVA and UVB protection". Better yet, consider limiting sun exposure, or wearing lightweight clothing and hats that cover and protect your skin.
