In terms of sunlight, only radiation in the form of visible light, UV-A and UV-B reaches the Earth's surface. Approximately 6% of the solar radiation that reaches the earth is UV rays. Normal window glass is impermeable to a large percentage of ultraviolet rays. This is particularly true of UV rays with short wavelengths (UV-B and UV-C); window glass does, however, allow UV-A through. Even glasses and optical components obstruct high-intensity short-wave ultraviolet rays. Intense UV radiation is capable of splitting organic bonds and does damage organic bonds in instances of extreme exposure (turbidity, embrittlement, decay). This is also called a "photo-oxidising reaction" The time before this takes effect can, according to scientific studies by D.J. Carlsson and D. M. Wiles, (Division of Chemistry, National Research Council of Canada "The Photo-oxidative Degradation of Polypropylene"), range from a few months to several years, depending on the geographic region, sun intensity, and the length of exposure each day for the product on display.