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Human Eye

Our eyes have...

Question

Our eyes have a .............. lens whose ......... can be changed at will with the action of ...... muscles.

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Solution

Correct answer is (a)

Our eye has convex lens which is composed of a fibrous, jelly-like material, Its curvature can be modified to some extent by the ciliary muscles which thus change its focal length. The ability of the eye lens to adjust its focal length is called accommodation. However, the focal length of the eye lens cannot be decreased below a certain minimum limit.

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Similar questions

Q. State whether true or false :

Our eyes have a convex lens whose focal length can be changed at will by the contraction and relaxation of cilliary muscles.

The change in focal length of an eye lens is caused by the action of the

(a) pupil

(b) retina

(c) ciliary muscles

(d) iris

Q. Fill in the blanks
The eye lens can change its focal length due to working of _________ muscles.

Q. The ciliary muscles can change the focal length of the eye lens. Find the ratio of focal lengths of the eye lens when it is focused on two different objects, one at a distance of 2 m and the other at a distance of 1m. The diameter of normal eye is 2.5 cm.

Q. The ciliary muscles of eye control the curvature of the lens in the eye and hence and alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of lens increases. That means, radius of curvature decreases and focal length decreases. For a clear vision, the image must be on the retina. The image distance is therefore fixed for clear vision and it equals the distance of retina from eye lens. It is about

25 c m

for a grown up person.
A person can theoretically have clear vision of an object situated at any large distance from the eye. The smallest distance at which a person can clearly see is related to minimum possible focal length. The ciliary muscles are most strained in this position. For an average grown up person, minimum distance of the object should be around

25 c m

.
A person suffering from eye defects uses spectacles (eye glass). The function of lens of spectacles is to form the image of the objects within the range in which the person can see clearly. The image of the spectacle lens becomes object for the eye lens and whose image is formed on the retina.
The number of spectacle lens used for the remedy of eye defect is decided by the power of the lens required and the number of spectacle lens is equal to the numerical value of the power of lens with sign. For example, if power of the lens required is

+ 3 D

(converging lens of focal length

100 / 3 c m

), then number of lens will be

+ 3

.
For all the calculations required, you can used the lens formula and lensmaker's formula. Assume that the eye lens is equiconvex lens. Neglect the distance between the eye lens and the spectacle lens.
A far sighted man cannot see objects clearly unless they are at least

100 c m

from his eyes. The number of the spectacle lenses that will make his range of clear vision equal to an average grown up person will be

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