Ultraviolet Light is a kind of solar radiation whose wavelength is between 200nmï½ž380nm, including three types: UVA (315nmï½ž380nm) UVB (280nmï½ž315nm) UVC 200nmï½ž280nm. Ultraviolet light accounted for about 13 in the reached earth surface solar radiation. Among them, UVA is about 97% and UVB is about 3%, so UVC is almost 0. So if the sunglasses can block UVA and UVB, they can block UV light. The extent of the sunglasses is determined by the maker and you can see it on the frames. So, a optician in a glasses shop can't determine whether the sunglasses block UV light, but they can tell you. Personally, I suggest you that do not buy many cheap sunglasses, maybe their blocking function are good as glasses with better quality.
No, they can't. The optician is the place selling all kinds of glasses, sunglasses or contact lenses. Therefore, when you buy sunglasses from the optician, you need to get to know their goods firstly. Remember to ask the technician the function and price of the sunglasses you wanna buy, which include the UV-blocking function of the sunglasses. Try to be more careful next time. Hope this will help you.