The structure of eye is just the same as a camera, which is designed according to eye. A transparent object, lens, is set in the window of eyes, namely between the pupil and the retina. All these light can be well reflected onto retina via the lens. However, if there is some problems in the reflecting, in that the images can not be focused onto the retina and some vision problems may occur. And people will suffer from blurriness. For this, some optical devices are needed, which can adjust the reflection to a proper position.
On the whole, the problem in the focus can be fallen into two types, either too much or insufficient. This leads to the emergence of two different glasses. The glasses lenses can be plus or minus, so as to adjust the coming angle of light properly. Convex and concave shapes are just the special designs of the two lenses. And how much can be light be adjusted is also determined by the strengths of these lenses.