An unmistakable popular icon and a timeless fashion
First of all, you should be wary about the cheap sunglasses which you get at any optical store. In the quest of saving money, you end up buying a cheap pair of prescription sunglasses which might be damaged, or considered disqualified after the production process.
These prescription sunglasses, even though they save money, are a serious danger to your eyes and vision. To understand the risk, you have to first know the mechanism of how a pupil works. When the brightness is high, the pupil contracts to protect from the glare, and the light is soft, it dilates for a clearer vision. Now, in case you are wearing a malfunctioning pair of prescription sunglasses, the tint of the lenses will reduce the light and intuitively the pupil will get dilated. But the faulty UV coating won't be able to block the harmful UV rays, thus more UV rays will reach the eye as your pupil is dilated. Thus, it is better to not a pair of prescription sunglasses at all than wearing a faulty one. Wearing discarded prescription glasses for a cheap price will make you end up in spending more money.
Secondly, a popular misconception among prescription sunglasses is that the tint of the lenses actually determines the ability to block the ultraviolet rays. The colour of the lenses do have a direct relation to the amount of shade it offers to your eyes, but the UV coating is generally same on the kind of colours. The colours which are known to offer the best shade are brown, light grey and coffee. The next ones which come in efficiency are green, blue or amber coloured lenses. It is known that the lighter the tinge of the lenses, the lesser efficiency they have to offer your vision shade. An easy way to check whether the tinge on your lenses are lighter than what is required is to stand in front of a mirror and if you can see your eyes through the lenses then you will need darker lenses.