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Class 8, Science, Chapter-13, Lecture-3, Human Eye (Notes)

ROLE OF DIFFERENT PARTS OF HUMAN EYE:

PARTS OF THE HUMAN EYE

ROLE

 Cornea

Protects the eye AND acts as lens

 Iris

 Changes the size of the pupil

 Pupil

 Controls the amount of light entering into the eye

 Crystalline Lens

 Refracts the light rays to obtain the image on the retina

 Ciliary muscles

 Changes the focal length of lens

 Retina

 Obtains a real, inverted image.

 Optic nerve

 Sends message from eye to brain

 Aqueous Humour

 Acts as lens

 Vitreous Humour

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

 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

PERSISTENCE OF VISION:

The ability of an eye to continue to see the image of an object for a very short duration after the object has disappeared from view is called persistence of vision.

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.
    Reason: High converging power of eye lens OR Elongation of eye ball
    Correction: By using spectacles containing Concave Lenses.
  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
    Reason: Low converging power of eye lens OR Shortening of eye ball
    Correction: By using spectacles containing Convex Lenses.