Ultraviolet light and tints – How to Choose Your Sunglasses
LightTravels are described as electromagnetic waves. They behave in the same way as waves in the ocean, with their peaks and valleys. The wavelength of the radiation they emit is what we use to identify the different colours. The wavelength is the distance between two adjacent peak or troughs on the wave. These measurements, which are very small because light has a very short wavelength, are taken in units called“nanometres” (nm)A nanometre is one billionthof a metre.
Consider the colours of a rainbow. Red is on the outside and blue/violet inside. The wavelength for red is 710nm. Green is 500nm and blue/violet 400nm. As we move from the red to the blue tinted polycarbonate
ends of the visible spectrum, the wavelength gets smaller. Ultraviolet is located beyond the violet band at wavelengths 400nm-100nm. However, the human eye can’t see ultraviolet light at wavelengths smaller than 400nm. We call it ultraviolet radiation.
While UV radiation is not visible to humans, many animals, insects, and birds can detect it. This helps them to locate prey, etc. Kestrels, for example, can sense the UV radiation field voles emit as they move about. This means that if there are many cross-sections of urine trails on the ground and the birds of prey see them, it is a good area to hunt for the voles.
One reason we are unable to detect UV light is because our crystalline lens has evolved to focus only on a small number of colours. This helps us see clearly, while if we can focus on a wider range of wavelengths, it could cause chromatic aberration which could result in a distortion of our vision, which could reduce image clarity.
Depending on the wavelength, ultra-violet radiation can also be broken down into three main bands.
1. UVC radiation – 100 – 290nm
The UVC radiation is absorbed by the ozone layer, which surrounds Earth as part the outer atmosphere. This prevents it from reaching the ground.
2. 2.UVB radiation – 290 – 320nm
UVB radiation is most dangerous because it can cause skin damage and eye irritation. UVB radiation is responsible forSnowblindnessThe radiation causes a photochemical reaction in cornea and conjunctiva. After a few hours, the cornea becomes swollen and the surrounding tissues become inflamed. Oedematous corneas can cause blurred vision and severe pain. Photophobia is a fear of light and eyelid spasms. It lasts usually for a few days and can be relieved with local anaesthetic drops (amethocaine 1%) to reduce pain and adrenalin drops 0.01 to relieve congestion.
UVB radiation cannot pass through glass so a simple lens would keep it from reaching your eyes. It can also be cut out completely by polycarbonate, which is a plastic used to make sunglass lenses and spectacles.
Many believe that prolonged exposure to UVB radiation may lead to cataract formation earlier than usual. It can also result in the development of pterygia. These are tissue growths that encroach on the cornea from the sides.
The intensity of UVB radiation depends on the time of the day. It is most intense between 10am to 2pm when the sun shines brightest. Due to the direct sun’s rays, UVB radiation is stronger at higher altitudes and more intense close to the Equator.
3. UVA radiation – 320 – 400nm
UVA radiation, unlike UVB which changes in intensity depending on the time of day and night, is more stable with less variation. It is also not blocked by glass and can still reach the eye. The crystalline lens in the human eye absorbs the majority of UVA radiation so it is not allowed to pass to the back. Although there is no evidence to suggest that UVA can cause damage to the eyes, scientific opinion remains divided.
Although UVA radiation was initially thought to be minimally harmful to skin, recent research has shown that UVA penetrates deeper layers of skin and causes more damage.