If the focal length of a lens is f then its reciprocal (1/f) is called the power of the lens. Thus
Power of the lens = p = 1/f
When f is measured in metres then power of the lens is in dioptres’ the unit of power “dioptre” is defined as the power of a lens of focal length one metre. Its symbol is ‘D’.
Because the focal length of a convex lens is positive (+f) therefore the power of a lens is also positive.
Whereas the focal length of a concave lens is negative therefore the power of a concave lens is negative. Opticians and eye specialists refer to the power of a lens instead of its focal length for their patients.
Interesting information
Some animals like fish has the ability to move their eye lenses forward or backward and hence are able to see the objects around them clearly.
