Human Eye and Defects of Vision PDF – Eye Defects Correction

Human Eye and Defects of Vision PDF – Eye Defects Correction

Human Eye

It is an optical instrument just like a photographic camera. It forms the real image of the object on retina of the eye. For normal eye, the range of vision is from 25 cm to infinity.

In other words, human eyes are the organs responsible for reacting to light and colours and forming images that are then interpreted by the brain. The biology of the eye permits light to enter through the pupil and be bent by a number of biological structures which are used to focus light onto the retina.

A cross sectional view of the eye with various structures labeled

Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea. The crystalline lens merely provides the finer adjustment of focal length required to focus objects at different distances on the retina. We find a structure called iris behind the cornea. Iris is a dark muscular diaphragm that controls the size of the pupil. The pupil regulates and controls the amount of light entering the eye. The eye lens forms an inverted real image of the object
on the retina. The retina is a delicate membrane having enormous number of light-sensitive cells. The light-sensitive cells get activated upon illumination and generate electrical signals. These signals are sent to the brain via the optic nerves. The brain interprets these signals, and finally, processes the information so that we perceive objects as they are.

Defect of Vision And Their Corrections 

Sometimes there are situations where the eye cannot produce a focused image formation onto the retina. And we examine how these eye defects are corrected using corrective eye-wear or laser eye surgery.

Myopia (Short Sightedness)

In this case, the person cannot see the distant object. Image is formed before the retina. Concave lens is used for correcting myopia.

Myopia or Nearsightedness

Causes of Myopia

  1. Elongation of eye ball along the axis.
  2. Shortening of focal length of eye lens.
  3. Over stretching of ciliary muscles beyond the elastic limit.

Remedy of Myopia

  • Concave (Diverging) lens is used.

Hyperopia or Hypermetropia or Longsightedness

In this case, the person cannot see near object. Image is formed behind the retina. Convex lens is used for correcting hypermetropia.

Hyperopia or Farsightedness

Causes of Hyperopia

  1. Shortening of eye ball along the axis.
  2. Increase in the focal length of eye lens.
  3. Stiffening of ciliary muscles.

Remedy of Myopia

  • A Convex (Converging) lens is used.

Presbyopia

In the old age, the power of accommodation of the eye lens decrease, therefore, neither near nor distant objects are clearly seen.

Causes of Presbyopia

  • Eye looses much of its accommodating power due to stiffening of ciliary muscles.

Remedy of Presbyopia

Presbyopia can be removed by using focal lenses(two separate lens or a bifocal lens).

Astigmatism

In this case, the curvature of cornia becomes irregular and image is not clear.This defect arises due to differences 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.

Remedy of Astigmatism

Cylindrical lens is used for correcting astigmatism.

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.

Eye Related Problems

Cataract

In this defect, an opaque, white membrane is developed on cornea due to which a person losses power of vision partially or completely. This defect can be removed by removing this membrane through surgery. In other words, a cataract is a clouding of the lens which prevents a sharp, clear image being produced. A cataract is arise because the lens is sealed in a capsule and as old cells die, they get trapped in the capsule, with time. This causes a clouding over the lens. This clouding cause results in blurred images.

Gloucoma

The eyes generate a clear fluid (aqueous humor) which fills the space between the cornea and the iris. This generated fluid filter out through a complex drainage system. This is the balance between the production and drainage of this liquid that determines the eyes intraocular pressure (IOP). Glaucoma is a disease which is caused by increased IOP usually resulting from a malfunction in the eye’s drainage system. Increased IOP can also cause irreversible damage to the optic nerves and retinal fibers and if left untreated can result in a permanent loss of vision.

Age-Related Macular Degeneration (ARMD)

It is a degenerative condition of the macula (the central retina). The reason of its cause is the hardening of the arteries that nourish the retina. This deprives the retinal tissue from the nutrients and oxygen that it need to function and causes a deterioration in central vision.

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 microscopic is given as

M = 1 + D/f where D = 25 cm, f = focal length of lens.

Compound microscope

It consists of two convex lenses co-axially fitted in a hollow tube. The lens facing the object is called objective and the lens towards the 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 telescope and Galilean telescope.

Astronomical telescope consists of two convex lenses placed co-axially in a hollow tube. The lens facing the object is called objective and the lens towards 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.

Defects of Vision and Their Correction PDF

Aspirants can view and download the defect of vision and their correction pdf by clicking on below link.

Defect of Vision and their Correction

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