Far point and near point in an Eye
Far Point and Near Point in an Eye
Far Point And Near Point: Although the power of accommodation of the eye is amazing, it has certain limitations. The focal length of the eye cannot adjust enough to form sharp images of objects kept beyond a certain point and closer than a certain point.
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The farthest point up to which the eye can see properly is called the far point of the eye. For the normal eye, the far point is at infinity. When viewing an object at the far point, the ciliary muscles are in their most relaxed state, and the crystalline lens is at its thinnest (Figure 2.27a). Thus, the eye-lens is at its maximum focal length when looking at an object at the far point. As an object is brought closer to the eye, the ciliary muscles contract. This increases the thickness of the crystalline lens and decreases the focal length of the eye-lens. This helps to form a sharp image on the retina. At a certain point near the eye, the ciliary muscles reach the limit of their contraction. If an object is brought closer still, the ciliary muscles cannot contract further. The closest point at which an object can be placed and seen clearly is called the near point of the eye. And, the distance of the closest point at which an object can be placed and seen clearly is called the least distance of distinct vision or the least distance of clear vision |
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When an object is placed at the near point (that is, at a distance equal to the least distance of distinct vision), the curvature and thickness of the crystalline lens are maximum and the focal length of the eye-lens is minimum.
.The near point of the eye varies from person to person and with age. At a very young age (below 10 years), the ciliary muscles are strong and flexible, and therefore, they can contract to a great extent. At this age, the near point may be as close as 7 to 8 cm from the eye. In old age, the ciliary muscles lose flexibility, and hence, the ability to contract to the required extent is lost.As a result, the near point reaches to 1 to 2 m, or even more. However, the standard value of the least distance of distinct vision for the normal human eye is taken as 25 centimetres.
Which of the following are correct : (a) The eye-lens is at its maximum focal length when looking at an object at the far point. (b) The eye-lens is at its minimum focal length when looking at an object at the far point.
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| Right Option : A | |||
| View Explanation | |||
When an object is placed at the far point, the curvature and thickness of the crystalline lens are ___________and the focal length of the eye-lens is ______________. | |||
| Right Option : A | |||
| View Explanation | |||
When an object is placed at the near point, the curvature and thickness of the crystalline lens are ___________and the focal length of the eye-lens is ______________. | |||
| Right Option : C | |||
| View Explanation | |||
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