Study Material and Notes of Ch 11 Human Eye and Colourful World Class 10th Science
Topics in the Chapter
• Introduction
• Parts of Human Eye
→ Cornea
→ Lens
→ Iris
→ Pupil
→ Retina
→ Far point
• Near point or Least distance of distinct vision
→ Accomodation
• Myopia (Near sightedness)
→ Causes of Myopia
→ Correction of Myopia
• Hypermetropia
→ Causes of Hypermetropia
→ Correction of Hypermetropia
• Presbyopia (Old age Hypermetropia)
→ Causes of Presbyopia
→ Correction of Presbyopia
• Advantage of the eyes in front of the face
Introduction
→ Human eye: The sense organ that helps us to see.
→ Located in eye sockets in skull.
→ Diameter of eye ball – 2.3 cm.
Parts of Human Eye
• Cornea: It is the outermost, transparent part. It provides most of the refraction of light.
• Lens: It is composed of a fibrous, jelly like material. Provides the focused real and inverted image of the object on the retina. This is convex lens that converges light at retina.
• Iris: It is a dark muscular diaphragm that controls the size of the pupil.
• Pupil: It is the window of the eye. It is the central aperture in iris. It regulates and controls the amount of light entering the eye.
• Retina: It is a delicate membrane having enormous number of light sensitive cells.
• Far point: The maximum distance at which object can be seen clearly is far point of the eye. For a normal adult eye, its value is infinity.
Near point or Least distance of distinct vision
→ The minimum distance at which objects can be seen most distinctively without strain.
→ For a normal adult eye, its value is 25 cm.
→ Range of human vision – 25 cm to infinity.
• Accomodation: The ability of the eye lens to adjust its focal length is called accommodation. Focal length can be changed with the help of ciliary muscles.
→ Focal length increases when Ciliary muscles get relaxed and lens get thin.
→ Focal length decreases when Ciliary muscles get contract and lens get thick.
Myopia (Near sightedness)
→ A myopic person can see nearby objects clearly but cannot see distant objects clearly.
→ Image is formed in front of retina.
Causes of Myopia
→ Excessive curvature of eye lens.
→ Elongation of eye ball
Correction of Myopia
→ It is done by using concave lens of appropriate power.
(ii) The far point (F) of a myopic eye is less than infinity.
(iii) Correction of myopia. The concave lens placed in front of the eye forms a virtual image of distant object at far point (F) of the myopic eye.
Hypermetropia (Far sightedness)
→ Affected person can see far objects clearly but cannot see nearby objects clearly.
→ The near point of the eye moves away.
→ Image is formed behind the retina.
Causes of Hypermetropia
→ Focal length of the eye lens becomes too long.
→ Eye ball becomes too small.
Correction of Hypermetropia
→ Use of convex lens of suitable power can correct the defect.
Presbyopia (Old age Hypermetropia)
→ It is the defect of vision due to which an old person cannot see the nearby objects clearly due to loss of power of accomodation of the eye.
→ The near-point of the old person having presbyopia gradually recedes and becomes much more than 25 cm away.
Causes of Presbyopia
→ Gradual weakening of ciliary muscles.
→ Diminishing flexibility of eye lens.
Correction of Presbyopia
→ Use of convex lens of suitable power.
→ Sometimes a person may suffer from both myopia and hypermetropia.
→ Such people require bifocal lens for correction.
Advantage of the eyes in front of the face
→ It gives a wider field of view.
→ It enhances the ability to detect faint objects.
→ It provides three dimensional view.