Posted on August 29 2015
Minus lenses work by the same general concepts of bending light as plus lenses. However, minus lenses achieve the opposite result for the wearer, extending the focal point rather than shortening it. Minus lenses are also a bit more complex than plus lenses because their shape, when worn in glasses, is neither exactly concave nor convex.
Recall that minus lenses are prescribed for people with nearsightedness or myopia. These people can see up close without a problem but have difficulty seeing distance because their eye bends light too sharply, causing the light waves coming into their eye to have a focal point in front of the retina. These people need a lens that will counteract the excessive refracting of their eye. They need a minus lens that will disperse the light slightly before it hits the eye. A minus lens is diverging, which means it will push the light waves away from each other. Then those light waves are refracted by the eye in such that the focal point exactly hits the retina.
Like with a plus lens, we can discuss a minus lens in relation to a prism. In a prism, the thickest part is the base and the narrowest part is the apex. Light waves are bent towards the thickest part. With a plus lens, the base is in the middle of the lens and the apex is at the edges, at the top and bottom of the lens. It is just the opposite with a minus lens. The thickest part (base) is at the top and bottom of the lens while the thinnest part (apex) is the middle. This causes the light waves to bend towards the thickest parts, at the top and bottom of the lens. In other words, the light waves bend away from each other. This would theoretically cause a minus lens to be concave (a plus lens is a convex shape). However, you don’t see glasses where the outer surface is concave. A concave lens would mean the outer surface of the lens would curve inward, towards the frames and eye. Instead, for a number of reasons, minus lenses are made with the outer surface convex and the inner surface (closest to the eye) concave.
Like plus lenses, minus lenses are measured in diopters. However, a minus lens will feature negative numbers, such as “-3.00.” Just as with math, negative numbers added together will result in a lower number. So if you combined a -3.00 diopter lens with a -2.00 lens, it would be the same diverging power as a -5.00 lens. The diopter amounts in plus and minus lens can be combined as well. If you were able to wear both -3.00 diopter lenses and +2.00 lenses, it would the same as a -1.00 diopter lens. However, keep in mind that a myopic person needs a minus lens. So a nearsighted person with a -3.00 prescription would see distance less well with a -1.00 lens.