How to know the power of spectacle for Nearsightedness or hypermetropia
Nearsightedness the condition, wherever someone will see close to objects clearly however distant objects area unit bleary usually is definitely corrected with prescription eyeglasses or contact lenses. Lenses are wont to correct myopia area unit concavo-concave in form. In other words, they are thinnest at the centre and thicker at the edge.
These lenses area unit is known as “minus power lenses” (or “minus lenses”) as a result of they cut back the focusing power of the attention. By doing this, minus lenses move the main target of sunshine in a very nearsighted eye from some extent before the membrane backwards thus it falls directly onto the surface of the retina. This shift corrects the bleary sense caused shortsightedness, restoring clear vision.
Myopia is the defect of the eye also known as nearsightedness. In this defect, a person can see the near objects clearly but cannot see the far point objects due to the enlarge of eyeball and image formed at in front of the retina, not on the retina. The concave lens requires to remove this defect.
If a person can see the objects up to d distance clearly in front of the eye then to calculate the power of the spectacle is
where negative power means concave lens and f is the focal length of a concave lens.