Modern eyeglasses are frames with two lenses worn in front of the eyes. The eyeglasses are usually braced by pads on the bridge of the nose and temples placed over the ears. Eyeglasses are normally designed for the purpose of vision correction, eye protection, viewing visual information or fashion and aesthetic values. Optical eyeglasses are designed mainly for vision correction.
Optical eyeglasses’ frames are commonly made from metal or plastic. And the lenses of optical eyeglasses can be made from glass or transparent plastic, such as CR-39 or polycarbonate. Whatever kind of materials the lenses made from, the principle of optical eyeglasses is similar.
The optical eyeglasses’ lens is an optical device with perfect or approximate axial symmetry which can converge or diverge the light.
Most optical eyeglasses are spherical lenses, the two surfaces of which are parts of the surfaces of spheres, with the lens axis ideally perpendicular to both surfaces. Lenses are classified by the curvature of the two optical surfaces. The two surfaces of the lens can be both convex, both concave, one flat and one convex, one flat and one concave, or with one convex and one concave. The last one, called convex-concave or meniscus, is the type of lens that is most commonly used in optical eyeglasses.
Optical eyeglasses can be either positive or negative, depending on the curvatures of the two surfaces. Negative optical eyeglasses have steeper concave surfaces and will be thinner at the centre than at the periphery. This type of optical eyeglasses is used for correcting nearsightedness. Conversely, positive optical eyeglasses have steeper convex surfaces and will be thicker at the centre than at the periphery, and it is for presbyopia.