JEE Main & Advanced Physics Ray Optics Defects in Eye

(1) Myopia (short sightness) : A short-sighted eye can see only nearer objects. Distant objects are not seen clearly.

(i) In this defect image is formed before the retina and Far point comes closer.

(ii) In this defect focal length or radii of curvature of lens reduced or power of lens increases or distance between eye lens and retina increases.

(iii) This defect can be removed by using a concave lens of suitable focal length.

(iv) If defected far point is at a distance d from eye then Focal length of used lens \[f=-d=-\] (defected far point)

(v) A person can see upto distance \[\to x\], wants to see distance \[\to y(y>x)\] so \[f=\frac{xy}{x-y}\] or power of the lens \[P=\frac{x-y}{xy}\]

(2) Hypermetropia (long sightness) : A long-sighted eye can see distant objects clearly but nearer object are not clearly visible.

(i) Image formed behind the retina and near point moves away

(ii) In this defect focal length or radii of curvature of lens increases or power of lens decreases or distance between eye lens and retina decreases.

(iii) This defect can be removed by using a convex lens.

(iv) If a person cannot see before distance  d  but wants to see the object placed at distance D from eye so \[f=\frac{dD}{d-D}\] and power of the lens \[P=\frac{d-D}{dD}\]

(3) Presbyopia : In this defect both near and far objects are not clearly visible. It is an old age disease and it is due to the loosing power of accommodation. It can be removed by using bifocal lens.

(4) Astigmatism : In this defect eye cannot see horizontal and vertical lines clearly, simultaneously. It is due to imperfect spherical nature of eye lens. This defect can be removed by using cylindrical lens (Torric lenses).