Light travels as electromagnetic waves, similar to the waves on the sea, with peaks and troughs. We specify the different colours of light by the wavelength of radiation they emit. (The wavelength is the distance between two adjacent peaks or troughs of that wave). As light has a very small wavelength, these measurements are extremely small and are measured in units called “nanometres” (nm), where a nanometre is one billionth of a metre!
Think of the colours of a rainbow, with red on the outer part of the curve and blue/violet on the inside. Red has a wavelength of 710nm, green is 500nm, and blue/violet is 400nm. So the wavelength is getting smaller as we pass from the red end of the visible spectrum through to the blue end. Ultra-violet lies beyond the blue/violet band, between wavelengths 400nm to 100nm, but the human eye cannot usually see light with a wavelength smaller than 400nm, making ultra-violet light invisible to us – so we call it ultraviolet radiation.
Although UV radiation is invisible to humans, many animals and insects can detect UV light, which they use in helping to find prey, etc. For example, kestrels can detect the UV light given off by the urine trails that field voles make as they move around, so from high up in the sky, when these birds of prey find many criss-crossing urine trails on the ground, this indicates a good place to hunt for the voles.
One possible reason why we cannot detect UV light is that if the crystalline lens in the human eye has evolved to only focus on a limited range of colours, this helps to give us a sharper image of what we see, whereas if we could focus over a larger range of wavelengths this could lead to chromatic aberration, a distortion of vision that would reduce image clarity.
Ultra-violet radiation can be divided into three main bands or groups, depending on wavelength :-
1. UVC radiation – 100 to 290nm
UVC rays are absorbed by the all-important ozone layer that surrounds the Earth as part of the outer atmosphere, thus preventing this radiation from reaching the ground.
2. UVB radiation – 290 to 320nm
UVB radiation is the most dangerous, as this is what can damag custom blue light blocking glasses e the eyes and cause sunburn to the skin. UVB is the cause of snow-blindness, where the radiation induces a photochemical reaction in the cornea and overlying conjunctiva after a few hours exposure, producing swelling of the cornea (oedema) and inflammation of the surrounding tissue. The swollen, oedematous cornea becomes cloudy – hence the blurred vision, and also very painful. There is photophobia (dislike of any exposure to light) and spasm of the eyelids. It usually lasts for a couple of days, and is only alleviated by using local anaesthetic drops, amethocaine 1%, to reduce the pain, and adrenalin drops 0.01% to relieve the congestion.
UVB radiation does not pass through glass, so a simple glass lens would prevent it from reaching the eye. Polycarbonate, a type of plastic sometimes used for spectacle and sunglass lenses, also cuts it out completely.
Some believe that regular exposure to UVB radiation can, over a period of time, contribute to cataract formation at an earlier age than normal, and may also cause the development of pterygia, which are growths of tissue that encroach over the cornea from the sides.
UVB radiation varies with the time of day, being most intense between 10am and 2pm, when the sun is at its brightest. It is