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Class 10, Science, Chapter-9, Lecture-7, Human Eye (Notes)

ROLE OF DIFFERENT PARTS OF HUMAN EYE:

PARTS OF THE HUMAN EYE

ROLE

1. Cornea

· Protects the eye AND acts as lens

2. Iris

· Changes the size of the pupil

3. Pupil

· Controls the amount of light entering into the eye

4. Crystalline Lens

· Refracts the light rays to obtain the image on the retina

5. Ciliary muscles

· Changes the focal length of lens

6. Retina

· Obtains a real, inverted image.

7. Optic nerve

· Sends message from eye to brain

8. Aqueous Humour

· Acts as lens

9. Vitreous Humour

· Provides shape to the eye. (lets the light pass through it without refraction)

10. Blind spot

 

WORKING OF HUMAN EYE:

A light-ray from a source passes through cornea and aqueous humour and gets refracted.
The refracted light-ray enters through the eye lens where it is finally refracted by very small amount so as to form a real inverted image on the retina.
The light sensitive cells present in the retina get activated and send electrical signals to the brain.

ACCOMMODATION:

The ability of an eye to focus on distant object and nearby object as well on the retina by changing the focal length of the eye lens is termed as accommodation.

RANGE OF VISION – from infinity to about 25 cm away from the eye

POWER OF ACCOMMODATION:

The maximum variation of the power of eye lens for focussing on near and distant objects is termed as the power of accommodation of the eye.

  • For normal vision – power of accommodation is 4 dioptres ( 4 D )

FAR POINT:

The farthest point from the eye which can be seen clearly is called the far point of the eye.

  • For normal vision – the far point is infinity.

NEAR POINT:

The nearest point up to which an eye can see clearly is called the near point of the eye.

  • For normal vision – the near point is 25 cm away from the eye.

LEAST DISTANCE OF DISTINCT VISION:

The minimum distance at which an object must be placed so that a normal eye may see clearly, is called the least distance of distinct vision.

  • For normal vision – the least distance of distinct vision is 25 cm.
  • It is denoted by D

CATARACT:

The disease in which the eye-lens becomes hazy due to formation of a membrane over it and leads to partial or complete loss of vision is called cataract.

Reason: Development of a membrane over the eye-lens.
Correction: Removal of the membrane by Cataract surgery.

REFRACTIVE DEFECTS OF VISION:

  1. SHORT SIGHTEDNESS:
    The defect of the eye due to which an eye cannot see the distant objects clearly but can see the nearby objects clearly is called short sightedness or Myopia.
    Reasons:
    In this disease, image is formed in front of the retina due to
    i) The excessive curvature of the cornea leading to high converging power of eye lens, OR
    ii) The elongation of the eyeball
    Correction:
    It can be corrected by diverging the rays by spectacles containing CONCAVE lens.
  2. LONG SIGHTEDNESS:
    The defect of the eye due to which an eye cannot see the nearby objects clearly but can see the distant objects clearly is known as long sightedness or Hypermetropia.
    Reasons:
    In this disease, image is formed behind the retina due to
    i) Very large focal length of the eye lens leading to low converging power of eye lens,
    ii) The shortening of the eyeball.
    Correction:
    It can be corrected by converging the rays by spectacles containing CONVEX lens.
  3. PRESBYOPIA
    The decrease in the power of accommodation of the eye of an old person due to the weakening of the ciliary muscles which leads to hypermetropia, or both myopia and hypermetropia is termed as presbyopia. 
    Reasons
    i) Weakening of ciliary muscles. 
    ii) Reduced flexibility of the crystalline eye lens. 
    Correction: It can be corrected by spectacles containing BIFOCAL lens.