Answer to Question #24123 in Optics for ismita bharti
Converging lenses have positive optical power, while diverging lenses have negative power. When a lens is immersed in a refractive medium, its optical power and focal length change.
For two or more thin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: P = P1 + P2. Similarly, the optical power of a single lens is roughly equal to the sum of the powers of each surface. These approximations are commonly used in optometry.
An eye that has too much or too little refractive power to focus light onto the retina has a refractive error. A myopic eye has too much power so light is focused in front of the retina. Conversely, a hyperopic eye has too little power so when the eye is relaxed, light is focused behind the retina. An eye with a refractive power in one meridian that is different from the refractive power of the other meridians has astigmatism. Anisometropia is the condition in which one eye has a different refractive power than the other eye.
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