##### Physics
General Knowledge

# Human Eye

## Human Eye

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 :
Elongation of eye ball along the axis.
Shortening of focal length of eye lens.
Over stretching of ciliary muscles beyond the elastic limit.
Remedy : Diverging lens is used.

2. Hyperopia or hypermetropia or longsightedness
A person suffering from hypermetropia can see the distant objects clearly but not the near objects.

Causes :
Shortening of eye ball along the axis.
Increase in the focal length of eye lens.
Stiffening of ciliary muscles.
Remedy : A converging lens is used.

3. Presbyopia
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 can not 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 rays 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

This is simply a convex lens of small focal length. The object to be enlarged is placed within the focus of lens.
Magnifying power of a simple microscope is given as
M = 1 + D/f
where, D = 25 cm, f = focal length of lens.

## Compound microscope

It consists of two convex lenses coaxially fitted hollow tube. The lens facing the object is called objective and the lens toward ^ eye is called eye piece.
» The aperture of objective is smaller than that of eye piece.
» Both the lenses are of smaller focal lengths. This increases the magnifying power of instrument.

## Telescope

Telescopes are used to view distant objects which are not visible to naked eye. Telescope can be divided as astronomical telescope, terrestrial telescn Galilean telescope.
» Astronomical telescope consists of two convex lenses placed coaxially in a hollow tube. The lens facing the object is called objective and the lens toward the eye is called eye piece.
» The objective has large aperture so that the rays from the object can be easily collected.
» The focal length of objective is larger than that of eye piece.

## Movie (Film) or Slide Projector

It projects magnified image of an object on a screen. The object (inverted) is placed between F and 2F of a projection lens (a convex lens) so that its real, inverted magnified image is formed between 2F and infinity on the other side i.e. the image is erect.