clip on eyeglasses are also called magnetic eyeglasses or clip on sunglasses. They are made of two layers of lenses. They are one type of eyeglasses that designed for people with poor vision. The inside layer of lenses are made of prescription to correct vision while the outside colored lenses use for prevent UV rays when you in sun. And indoor, you can also take off the out side layers of lenses.
clip on eyeglasses have two parts, one is regular glasses for vision correction and the other is sunglasses for eye protection. You can turn up the clip ons when you are indoors and only see through the prescription ones so that you can see things clearly indoors. When you go outdoors, you just turn down the clip ons, which not only allow you to see clearly but also protect your eyes from harmful UV rays.
Clip on glasses are the type of glasses that come with two parts, one for vision correction and the other for sunlight blocking. Usually the base pair is one with prescribed lenses and the extra pair pure shades. The magnatics or clips on the frames of the base pair make it possible to clip the extra pair upto the base pair whenever protection from sunlight is needed.