The Human Eye and The Colourful World
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.
Myopia (Near sightedness)
A myopic person can see nearby objects clearly but cannot see distant
- Image is formed in front of retina. Causes of Myopia
- Excessive curvature of eye lens
- Elongation of eye ball
Use of concave lens of appropriate power.
Hypermetropia (Far sightedness)
Affected person can see far objects clearly but cannot see nearby objects
- 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.
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
- Gradual weakening of ciliary muscles.
- Diminishing flexibility of eye lens. Correction
- Use of convex lens of suitable power.
- Sometimes a person may suffer from both myopia and hypermetropia.
• Such people require bifocal lens for correction.