Visible light does not seem to cause cataract, although claims have been made that individuals from white races living for long periods in the tropics can show a higher incidence of cataract. In practice, this is not easy to confirm. In spite of public misapprehension, ultraviolet light probably does not cause cataract either, because the shorter wavelengths fail to penetrate the globe. These shorter wavelengths beyond the blue end of the visible spectrum can produce a dramatic superficial burn of the cornea, which usually heals in about 48 h. This injury,which is typified by "snow blindness" and "welders' flash", will be discussed in Chapter 15. Prolonged doses of infrared rays can produce cataract; this used to be seen occasionally in glassblowers and steel workers, but the wearing of goggles has now more or less eliminated this. X-rays and gamma rays can also produce cataracts, as was witnessed by the mass of reports that followed the explosion of the atomic bombs at Nagasaki and Hiroshima in Japan. Radiation cataract is now seen following whole-body radiation for leukaemia but the risk is only significant when therapeutic doses of X-rays are used.
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