Human Eye- Part 4

Q) When the eye is looking at nearby objects, the eye is said to be accommodated. Describe it? 

 -> When the eye is looking nearby objects, the ciliary muscles of the eyes contract. The contracted ciliary muscles make the suspensory ligaments loose. As a result of this, they stop pulling the eye lens and eye lens become thick (or more convex). 

    The thick eye lens has small focal length but its converging power is large. Since the converging power of eye lens increases, So the thick eye lens can convert the diverging light rays coming from the nearby object to form an image on the retina of the eye. when the eye lens become more convex to focus the nearby objects, then the eye is said to be accommodated. 

Defects of vision -> The ability to see is called vision or eyesight. Sometimes the eye of a person can’t focus the image of an object on the retina properly. 

    In such cases, the vision of person becomes blurred and he cannot see either the distant objects or nearby objects clearly, then that person is said to have a defects of vision or defects of eye. 

There are three common defects of vision:- 

* Myopia (short-sightedness or near sightedness) 

* Hypermetropia (long-sightedness or far sightedness) 

* Presbyopia 

  Note:- These defects of vision are called refractive defects of vision because they are caused by the incorrect refraction of light rays by the eye lens. these defects of vision can be corrected by using suitable spherical lenses and these lenses are usually used in the form of spectacles.

(i) Myopia- The defect of eye in which a person can see nearby objects clearly but can’t see the distant objects clearly is called myopia. The far point of myopic eye is less than infinity.

     Myopia is caused due to:- 

 

 Correction -> Since a concave lens has an ability to diverge incoming rays. So, Myopia is corrected by using concave lenses. when a concave lens of suitable power is placed in front of the myopic eye then the parallel rays of light coming from the distant object are first diverged by the concave lens. 

    Due to this, the concave lens forms a virtual image of the distant object at the far point (F) of the myopic eye. Since the rays of light now appear to be coming from the eyes for point ‘F’, they can be easily focus by the eye lens to form an image on the retina.

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