There are 3 types of UV rays classified according to the wavelength:
The first type is UV-C (the wavelength is shorter than 280nm) which can be absorbed by ozonosphere and thus will not cause any harm to the human body generally.
The second type is UV-B (the wavelength is between 280 to 320nm) which can result in keratitis and skin cancer. This UV-B can cause serious radiation harm includs foreign body sensation, pain, photophobia, blepharospasm, tearing, corneal epithelium injury, the decrease of corneal sensation and so on.
The third type is UV-A (the wavelength is between 320nm to 380nm). It can be absorbed by the lens and has the chemical reaction and then cause protein denaturation and cloudiness. In this way, early cataract comes into being.
Therefore, people who suffer from cataract need to protect their eyes from UV.
According to some researches, one extra hour of sunbath a day can increase 10% of the risk of cataract in a year. And people who work outside have the risk of cataract as much as 3 times than average people. If there is nothing equipped to block the glare and UV, the eyes would be hurt by the rays for they would enter directly in the eyes. Long-time exposure in the strong UV rays can easily become one of the inducing factors.
People suffering from glaucoma should not wear sunglasses
People suffering from glaucoma feel comfortable in bright place; however, once putting on sunglasses, their eyes would be swollen and even has the condition intermittent emporary blindness. This is because when in the dim light, people wearing sunglasses would experience pupil dilation, and in this moment, the intraocular pressure of the glaucoma sufferers would increase due to the chamber angle closure, and thus cause the symptom of swollen eyes and emporary blindness. When the light become bright, the intraocular pressure would decrease and the eyes turn to be normal again. Therefore, people suffering from glaucoma should not wear prescription sunglasses (http://www.finestglasses.com/