Presbyopia is vision defect when the ability of accommodation of the eye usually decreases with ageing. For most people the near point gradually recedes away. They find it difficult to see nearby objects clearly and distinctly.
This happens due to gradual weakening of ciliary muscles and diminishing flexibility of the eye lens. This effect can be seen in aged people. Sometimes a person may suffer from both myopia and hypermetropia with ageing .
To correct this type of defect of vision we need bi-focal lenses which are formed using both concave and convex lenses. Its upper portion consists of the concave lens and lower portion consists of the convex lens.
If you go to an eye hospital to get tested for vision defects, the doctor gives you a prescription that contains some information regarding type of lens to be used to correct vision.
• Have you ever observed details in the prescription?
You might have heard people saying “my sight is increased or decreased”.
• What does it mean?
Usually doctors, after testing for defects of vision, prescribe corrective lenses indicating their power which determines the type of lens to be used and its focal length.
• What do you mean by power of lens?
The degree of convergence or divergence of light rays that can be achieved by a lens is expressed in terms of its power
The reciprocal of focal length is called power of lens
Let ‘f’ be the focal length of lens.
Power of lens P = 1 / f(in m); P = 100 / f (in cm)
The unit of power is dioptre
It is denoted by the letter ‘D’.
Doctor advised to use 2D lens. What is its focal length?
Solution:Given that power of lens P = 2D
Using, P = 100 / f (in cm); 2 = 100 / f
Therefore, f = 100/2 = 50 cm.
The lens has focal length, f = 50 cm.