Ultraviolet (UV) light and rays cannot be seen by the human eyes. This is because UV light has shorter wavelengths than visible light. Although much has been talked about in regards to UV rays causing damage to the skin leading to sunburns and prolonged and repeated exposures leading to incidences of skin cancer, only a few of us are aware of the damage UV rays can do to our vision. However, there is no need for panic since only some ultraviolet waves from the sun penetrate Earth's atmosphere; most of them are blocked from entering by various gases like ozone. (Source: National Aeronautics And Space Administration, NASA; Report Title: The Electromagnetic Spectrum; NASA Official: Ruth Netting; Last Updated: March 27th, 2007; URL: http://science.hq.nasa.gov/kids/imagers/ems/uv.html)
The other ways UV rays can cause damage to vision include temporary blindness, hazy vision, development of cataract and pain. So, is there a way to know how much UV radiation is safe for us?
Fortunately, yes. Scientists have devised an indexing system that indicates how long a person can expose themselves to the sun’s light on a given day before the possibility of damage sets in. The words "on a given day" mean that UV radiation intensity is not the same every day and may differ depending on:
• Cloud spread
• Thickness of ozone layer
• Time of day
• Earth’s surface characteristics
• Distance from Equator
The UV index is an international system of measurement of the strength or intensity of the UV radiation at around noon and point on the globe. It is measured in watts per meter square of Earth surface or w/m2. The measured values are translated to absolute numbers which are easy for people to read and understand the implications of.
Values range from 0 to 11+. Zero radiation essentially means night time and no radiation from UV rays and 11+ indicates extreme radiation and risk of harm from unprotected sun exposure; this is especially common around noon. Here is a look at what the index readings mean:
0-2: Low Risk – Use sunscreen if there is snow or you are in a place with less greenery or are very fair skinned.