Spherical Lenses

Lens shapes are designed to use refraction to correct for vision defects. A convex lens is used to correct for farsightedness. A convex, or converging lens, can form a variety of types and sizes of images depending on where the object is located relative to the focal point and center of curvature. When the object is closest to the convex lens, the image is virtual, upright and larger than the object. Moving the object farther from the lens when the object is at the focal point there is no image. Moving the object farther still, when the object is between the focal point and the center of curvature, the image is real, inverted and larger than the object. As the object moves even farther away, when the object is at the focal point, the image is real, inverted and the same size as the object. Moving the object beyond the center of curvature, the image is still real and inverted, but is not smaller than the object. A concave lens is used to correct for nearsightedness. In contrast, the concave, or diverging lens can produce only one type of image. The images produced by a concave lens are always virtual, upright and smaller than the object. Each type of lens has three principle rays – parallel, focal and center rays – that can be used to locate the image.