Why is it that only "expensive sunglasses" are the only ones that actually protect you?
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M$2 Answers
Fundamentally sunglasses need to totally block (100%) UVA and UVB sunlight in order to protect your eyes. They need to do this even at the extents of your vision so the rims should also effectively shield your eyes.
Secondarily they may contain tinting to block the Blue spectrum and make it easier to see clearly. (The eye is most sensitive to light in the yellow/green spectrum and Blocking high intensity blue light is is also good for your eyes).
Very cheap sunglasses do not block UV light - instead they just block light with dark plastic. In this way they can be easier to spot. If the sunglasses are very dark then they MAY not be using a UV blocker and just be relying on lowering the amount of light coming in. These are not good at all - in fact your irises will dilate to let more light in (because its so dark) and so you may be exposing yourself to more UV than without them.
There appears to be no correlation between price and safety (see ref below) unfortunately.
In different countries there are different rules for UV labeling on sunglasses. Your best bet is to find out how sunglasses are labeled and to make sure the ones you buy have UV protection.
Places like New Zealand and Australia which have very high amounts of UV reaching the ground (more so than in the USA) have good guidelines and labeling practices - especially for child products.
So pick sunglasses that are warm in tone, surround the eye so your peripheral vision is covered and are not too dark. Avoid ones without uniform tint or visual rippling as they might indicate low quality. But labeling is your only safe way of determining that 100% of UV A/B light is blocked.
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M$see: http://www.hidalgos.com/pages/Selecting-Sunglasses.html
You can leave an optional "tip" with Mahalo's virtual currency, Mahalo Dollars. If you are asking a difficult question that might require some research, or if you'd like a wide variety of feedback, a higher tip often leads to more answers to your question.
M$
Cataract formation has been linked to UV light. If the sunglass provider does not offer visible, infrared and UV ultraviolet light transmission levels, you have no way of knowing what you are buying, regardless of price. Price alone is no longer an indication of quality or protection level.
See: http://www.hidalgos.com/pages/Selecting-Sunglasses.html
