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Note on Human Eye

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Accommodation:

Accommodation is defined as the ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of its lens.

Far point:

The far point of an eye is defined as the farthest point from the eye which can be seen clearly. The far point of human eye is at infinity.

Near point:

The near point of an eye is defined as the nearest point from the eye up to which eye can see objects clearly. The nearest point of normal human eye is 25 cm from eye.

Defects of vision of eye:

  1. Long sightedness (Hypermetropia)

    Hypermetropia / long-sightedness is the defect of vision in which the eye cannot see the nearby objects clearly but can see the distant objects clearly.

    Causes of Hypermetropia

    1. Shortening of theeye ball
    2. Increment of focal length
  2. Shortsightedness (Myopia)

    Myopia is the defect of vision in which an eye cannot see the distant objects clearly but can see nearby objects clearly.

    Causes of myopia

    1. Elongation of theeye ball.
    2. Decrement of focal length.

  • Conjunctive, ciliary body, aqueous, iris, cornea, pupil, lens, optic nerve, macula, retina, sclera, vitreous, etc. are the parts of human eye.
  • Accommodation is defined as the ability of an eye to focus the distant objects as well as the nearby objects on the retina by changing the focal length of its lens.
  • Far point of an eye is defined as the farthest point from the eye which can be seen clearly. The far point of human eye is at infinity.
  • Near point of an eye is defined as the nearest point from the eye upto which eye can see objects clearly. The nearest point of normal human eye is 25cm from eye.
  • Defects of vision of eye: Long sightedness (Hypermetropia) and Shortsightedness (Myopia).
  • The causes of hypermetropia are increment of focal length, shortening of eye ball etc.
  • The causes of myopia are decrement of focal length, elongation of eye ball etc.
.

Very Short Questions

The near point is 25 cm and the far point is infinity. The nearest point up to which an object can be seen clearly is known as near point of vision. Similarly, the farthest point up to which objects can be seen clearly is known as far point of vision.

The defect of eye due to which the image of distant or nearer objects is not formed at retina is known as defects of vision. Hence, the object appears un-clear.

If thecilliary muscle does not function at all, the focal length of the eye lens can not be changed. If an eye cannot focus the light rays coming from the objects, the vision of the eye cannot see the objects clearly only at a particular distance. The person with such defective eye can neither see the objects at near distance nor at far distance. Thus, the eyes have both types of defects. They are: -

  • Short-sightedness
  • Long-sightednes

The defect of vision in which a person cannot see the distant objects clearly is known as short-sightedness or myopia. The image of the distant objects is formed in front of retina. The main causes ofshort-sightedness or myopia are as follows: -

  • The size of the eye ball is too large or long.
  • The focal length of eye lens is too short.

The defect of vision in which a person cannot see the near objects clearly is known as long-sightedness or hypermetropia. The image of the near objects is formed in behind the retina. The main causes of long-sightedness or hypermetropia are as follows: -

  1. The size of the eye ball is too small or short.
  2. The focal length of eye lens is too long.

For an eye with short-sightedness, the light rays coming from distant objects should be diverged to retina which is possible only by using concave lens as concave lens is a diverging lens. Therefore, convex lens, being converging lens cannot be used to correct the defect of short-sightedness.

The two differences between eye and camera are as follows: -

Eye Camera
The focal length of eye lens can be changed by the power of accommodation. The distance between the eye lens and the retina is fixed. Hence, the focusing is done by changing the focal length of the lens. The focal length of the convex lens is used in a camera is of fixed type. Hence, adjusting the distance between the lens and the film does focusing.
Image is formed on the retina. Image is formed on the film in camera.

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  • The change in focal length of an eye lens to focus the image of objects at varying distances is done by the action of _______.

    ciliary muscle


    blind spot


    retina


    pupil


  • Hypermetropia is rectified by using ______.

    convex lens


    bi-focal lena


    plane lens


    concave lens


  • Myopia is rectified by using ______.

    plane lens


    concave lens


    bi-focal lena


    convex lens


  • The least distance of distinct vision for infants is ______.

    5cm - 8cm
    3cm - 6cm
    5cm - 10cm
    25cm - 30cm
  • Image is formed on ______.

    pupil


    ciliary muscle


    blind spot


    retina


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