Least distance of distinct vision is 25 cm.
Defects of human eye
and the remedies:
1.
Myopia or short sightedness: A
person suffering from myopia can see the near objects clearly while far objects
are not clear.
Causes: (i)
Elongation of eye ball along the axis.
(ii)
Shortening of focal length of eye lens.
(iii)
Over stretching of ciliary muscles beyond the
elastic limit
Remedy: diverging
lens is used.
2.
Hyperopia or hypermetropia or long sightedness: A person suffering from
hypermetropia can see the distant objects clearly but not the near objects.
Causes: (i)
Shortening of eye ball along the axis.
(iii)
Increase in the focal length of eye lens.
(iv)
Stiffening of ciliary muscles.
Remedy: a converging lens issued.
3.
Presboypia: this defect is generally
found in elderly person Due to stiffening of ciliary muscles, eye looses much
of its accommodating power. As a result distinct as well as nearby objects
cannot be seen. For its remedy two separate lens or a bifocal lens is used.
4.
Astigmatism: This defect arises due
to difference in the radius of curvature of cornea in the different planes. As
a result says from an object in one plane are brought to focus by eye in
another plane. For its remedy cylindrical lens is used.
There are two kinds of vision cells in the retina. They are
called rods and cones on account of their peculiar shape. Rods decides the
intensity of light where as cones distinguish colour of light.
Simple microscope:
