Converging Vs. Diverging
Converging lenses are often referred to as plus lenses or positive lenses and have at least one convex surface. Some have two convex surfaces and others have one convex surface and one flat surface. Regardless of style or shape, converging lenses are thicker in the middle than on the sides. They allow light waves to penetrate the thick middle of a lens at a single point before diverging to produce an inverted image. Converging lenses are often used to correct hyperopia and presbyopia.
Diverging lenses, also called negative or minus lenses, have at least one concave surface and are thinner in the middle and wider on the edges. Diverging lenses allow light waves to spread out once they penetrate the lens and are often used to treat myopia.
Diverging lenses are used to help correct myopia.
Spherical Vs. Cylindrical
Both converging and diverging lenses can be either spherical or cylindrical. Spherical lenses are more common and have two completely symmetrical spherical surfaces, causing light waves that are parallel to the lens axis. Cylindrical lenses are uniform and cylindrical in shape, hence the name, and are used to correct unequal refraction from the cornea in refractory issues such as astigmatism.
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