如果室外陽光刺眼，你可能會不假思索地帶副太陽鏡出門。但如果你要買副新的，那就得考慮很多因素了——是去折扣店還是商場專柜？到了店里以后，你又會被眾多的選擇弄的眼花繚亂。這篇文章就將帶你揭開太陽鏡的謎底，教你如何挑選太陽鏡。

A pair of sunglasses seems so simple -- it's two pieces of tinted glass or plastic in some sort of plastic or metal frame. How much more straightforward can something get? It turns out that there are many different things you can do with two pieces of glass, and these things can have a big effect on you when you use the lenses. As you will see in this article, there really is a difference between the various sunglasses you'll find out there.

There are four things that a good pair of sunglasses should do for you:

? Sunglasses provide protection from ultraviolet rays（紫外線） in sunlight. Ultraviolet (UV) light damages the cornea（角膜）and the retina（視網膜）. Good sunglasses can eliminate UV rays completely.

? Sunglasses provide protection from intense light.

When the eye receives too much light, it naturally closes the iris. Once it has closed the iris as far as it can, the next step is squinting（斜視）. If there is still too much light, as there can be when sunlight is reflecting off of snow, the result is damage to the retina. Good sunglasses can block light entering the eyes by as much as 97 percent to avoid damage.

? Sunglasses provide protection from glare（強光）.

Certain surfaces, such as water, can reflect a great deal of light, and the bright spots can be distracting or can hide objects. Good sunglasses can completely eliminate this kind of glare using polarization (分化).

? Sunglasses eliminate specific frequencies of light.

Certain frequencies of light can blur vision, and others can enhance contrast. Choosing the right color for your sunglasses lets them work better in specific situations.

When you buy a pair of cheap sunglasses, you often give up all of these benefits and can even make things worse. For example, if your sunglasses offer no UV protection, you increase your exposure to UV rays. The cheap sunglasses block some of the light, causing your iris to open to allow more light in. This lets in more of the UV light as well, increasing the damage UV light can cause to the retina. So there is a difference. Buying the right pair of good sunglasses for the conditions in which you use them gives you maximum protection and performance.

（來源：howstuffworks.com 實習生吳昭文）

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