How sunglasses work, the spectral composition of sunlight

Talking about sunglass lenses, here are some notes on the spectral composition of sunlight.The solar radiation spectrum includes radiations in a band between 190 to 2300 nanometres (nm)*. The human eye perceives the band between 400 and 780 nm.

The invisible spectrum: ultraviolet (UV) and infrared (IR) radiations

UV is high-energy radiation in the 190-400 nm band. They are dangerous for all biological tissues and in particular for the eye which does not "see" them since they do not reach the retina.This type of radiation is usually divided into three intervals:

UVC (190-280 nm): they are blocked by the ozone layer and hence not reaching the ground level·
UVB (280-320 nm): they reach the ground level and are absorbed by the cornea and the crystalline lens of the eye. They can be dangerous even after brief exposures·
UVA (320-400 nm): they penetrate to ground level to greater extent than UVB. These radiations pass through the cornea and are absorbed by the crystalline lens protecting the retina.

And furthermore:-

the intensity of UV rays increases one time every 10 meters of elevation above the sea level- it is 180 times more intense on the beach because of the great reflection caused by the sand
- vulnerability to UV rays increases with aging
- overexposure to UV rays causes the eye to adjust slower to the dark

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