In order to block harsh sunlight in hot summer days, and to keep off the had glare from snow fields, people would usually choose to wear sunglasses. However, one demerit of sunglasses is that the wearers can not see things clearly indoors, especially those wearing prescription sunglasses, for they could neither take off the sunglasses nor put them on for clear vision. Then how to get a resolution? With the development of science and technology in sunglass manufacture, people in modern times have put the axe in the helve. Transitions sunglasses, which have a magic power to change the color of the lens according to the change of sunlight intensity outside, are able to off a clear vision both indoor and outdoor.
The secret lies in the lenses, where argentic chloride and silver bromide, etc., are added. Those items are able to get chemical radiations: 2Ag=2Ag+X2. The argentic chloride can be resolved into silver and halogen, when encountering strong sunlight, or more specifically speaking ultraviolet radiation. The silver atoms are little black pellets, which when distributed uniformly, can darken the lenses. Similarly, when the light weakens, the chemical radiations reverses: 2Ag+X2=2Ag, and the lenses turn to transparent again.
Nevertheless, not everyone is advisable to wear transitions sunglasses. To wear them blindly would lead to sever harm to eyes. There are four groups of people inadvisable to wear transitions sunglasses, and they are:
People with excessive myopia. They originally view faintly, and if transition lenses are applied for them, they would feel rather hard to see characters, and even objects in vision clearly.
People with both myopia and astigmatism. They are probably to see things clearly in strong light, and then, the silver halide of transition lenses would always in a state of deterioration. As a result, it is impossible to have a clear vision easily for the wearers. Moreover, the pupils of the wearers would be in dilation state always, which is rather bad for eye health.
People with presbyopia. The reason why they are not transitions sunglass persons is the same with that of the people with excessive myopia.
People suffering from eye diseases, like cataract and glaucoma, etc.. Transitions sunglasses would lead to the worsening of their sickness, for they pose stress on their eyes.