<Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (January 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> The dioptre can also be used as a measurement of curvature equal to the reciprocal of the radius measured in metres . For example, a circle with a radius of 1 / 2 metre has a curvature of 2 dioptres . If the curvature of a surface of a lens is C and the index of refraction is n, the optical power is φ = (n − 1) C. If both surfaces of the lens are curved, consider their curvatures as positive toward the lens and add them . This will give approximately the right result, as long as the thickness of the lens is much less than the radius of curvature of one of the surfaces . For a mirror the optical power is φ = 2C . </P> <P> The magnifying power V of a simple magnifying glass is related to its optical power φ by </P> <Dl> <Dd> V = 0.25 m × φ + 1 (\ displaystyle V = 0.25 \ \ mathrm (m) \ times \ varphi + 1). </Dd> </Dl>

What is the range of power of the normal eye