Ok, you know how in the sporting stores they sell unknown brands of sunglasses for $20? They look just like the famous brands (Oakley) but for a fraction of the price.

Is it true those sunglasses are bad for your eyes? A science prof once told me never let your kid wear $5 sunglasses because they do not contain the proper elements in the lenses. He said they make it seem darker for your eye so that your eye muscles relax and therefore it is very dangerous because the lenses do not block the deadly rays of light.

Is this true or did I misunderstand.

cliff notes: Are $20 sunglasses dangerous for your eyes because they do not contain some elements of safety in the lenses that $200 sunglasses have?