EYE LENS: The eye lens is composed of a fibrous, jelly – like material.
The change in the curvature of the eye lens can thus change its focal
length.
IRIS: Iris is a dark muscular diaphragm that controls the size of the pupil.
PUPIL: The hole in a circular diaphragm is called pupil. The pupil regulates and
controls the amount of light entering the eye.
CORNEA: Light enters the eye through a thin membrane called the cornea. It is
also called window of the eye.
The eyeball is approximately spherical in shape with a diameter of about 2.3 cm.
RETINA: Human eye lens system forms a light – sensitive screen called retina.
OPTICAL NERVE: Optical nerve is formed by the nerve fibers coming from the
retina. It carries electrical signals to the brain.
ACCOMMODATION: The ability of the eye lens to adjust its focal length is
called accommodation.
DEFECTS OF VISION: Defects of vision is the loss of power of
accommodation of human eye.
OR
The abnormality of human eye is called defects of vision.
MYOPIA OR NEAR SIGHTEDNESS OR SHORT SIGHTEDNESS: A person
with myopia can see nearby objects clearly but cannot see distant
objects distinctly (clearly).
Nearby object far object
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CAUSES: 1) Excessive curvature (High converging) of the eye lens or
focal length is too short 2) Elongation of the eyeball or eyeball is too
large.
REMEDY: Myopic eye can be corrected by using concave lens of
suitable power.
HYPERMETROPIA OR HYPEROPIA OR FAR SIGHTEDNESS OR LONG
SIGHTEDNESS: A person with hypermetropia can see distant objects
clearly but cannot see nearby objects distinctly (Clearly).
Far object nearby object
CAUSES: 1) The focal length of the eye lens is too long or low
converging power of eye lens.
2) The eyeball has become too small.
REMEDY: Hypermetropia can be corrected by using a convex lens of
appropriate power.
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PRESBYOPIA: The power of accommodation of the eye usually
decreases with ageing.
It arises due to the gradual weakening of the ciliary muscles and
diminishing flexibility of the eye lens, a person may suffer from
myopia or hypermetropia or both myopia and hypermetropia. Such
people require concave lens or convex lens or bifocal lenses
respectively.
In bifocal lens, the upper portion consists of concave lens and lower
part is a convex lens.
Cataract: The crystalline lens of people at old age becomes milky and
cloudy. This condition is called cataract. This causes partial or
complete loss of vision. It is possible to restore vision through a
cataract surgery.
The wavelength of red colour is the longest and the wavelength
of violet is the shortest.
If the wavelength of a colour is large, the speed of the colour is
also large.
Dispersion: The phenomenon of splitting white light into its seven
constituent colours when it passes through any transparent medium (such as
glass prism) is called dispersion of white light.
Spectrum: The band of seven colours after dispersion of white light is
known as spectrum of white light.
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