Accommodation: Accommodation is the ability or property of the eyelens due to which it can change its curvature or focal length so that images of objects at various distances can be formed on the same retina. The focal length of the eyelens is automatically changed with the help of ciliary muscles as follows:
(i) Viewing far off objects: When the ciliary muscles are completely relaxed, the eyelens is thin and its focal length is maximum (equal to distance between eyelens and retina). The rays coming from the distant object are parallel to each other and they are focussed at the retina as shown in Fig.(a).
Fig. Accommodation of eyelens (a) Focussing parallel rays from infinity i.e., far point (b) Focussing rays from near point N.
(ii) Viewing nearby objects: When we look at a nearby object, the ciliary muscles contract, the eyelens bulges out and becomes thick and its focal length is reduced. This focusses the light from the nearby object on the retina, as shown in Fig. (b).
Define the following terms and give their values for a normal eye:
(i) Range of normal vision.
(ii) Least distance of distinct vision.
(iii) Near point of the eye.
(iv) Far point of the eye.
(v) Power of accommodation.
(a) What is myopia? State two reasons due to which this effect is caused.
(b) Draw a diagram to show the formation of image of a distant object by a myopic eye. How can such an eye defect be remedied?
Or
State two main causes of a person developing near sightedness. With the help of a ray diagram, suggest how he can be helped to overcome this disability.
