Common Defects of Image Formation

Emmetropia - Normal

• The emmetropia of the eye is a state in which the eye is relaxed and is focused on an object more than 6m away, where the light rays comming from an object are focused on the retina witout effort.
• An emmertropic eye does not need a corrective lens to focus the image.

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 Hyperopia

• Hyperopia is farsightedness condition where the eye ball is too short.
• The retina lies in the focal point of the lens, and the light rays have not yet come into focus when they reach the retina.
• Causes the greatest difficulty when viewing nearby objects.
• Corrected with convex lens, which cause lays rays to converge before they enter the eye.

Image from wikipedia https://www.britannica.com

   Myopia

• This is a near sightness condition in which the eye ball is too long.
• Light rays come into focus before they reach the retina and begin to diverge again by the time they fall on it.
• Corrected with concave lenses, which cause light rays to diverge slightly before entering the eye.
• Those with myopia see nearby objects clearly but distant objects appear blurred.

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 Presbyopia

• This is the reduced ability to accommodate for near vision with age because of declining flexibility of the lens.
• This results in difficulty in reading and close hand work.
• Corrected with bifocal lenses.

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Astigmatism

• This is the inability to simultaneously focus light rays that enter the eye on different planes.
• Focusing on vertical lines, such as the edge of a door, my cause horizonal lines, such as a tabletop to go out of focus.
• This is caused by a deviation in the shape of the cornea so that it is shaped like the back of a spoon rather than a sphere.
• Corrected with cylindrical lenses, which refract light more in one plane than another.

Image from wikipedia https://www.britannica.com

Common Causes of Blindness

The two most common causes of blindness are cataracts and glaucoma.

Cataract

• Cataract is the clouding of the lens.
• It occurs as the lens thickens with age, it is a common complicatio of diabetes mellitus.
• It cause vision to appear milky or as if one were looking from behind a water fall.
• This condition may also stem from heavy smoking and exposure to UV radiation of the sun.
• This can be treated by replacing the natural lens with a plastic one. The implanted lens improves vision almost immediately, but glasses still maybe needed for near vision.

Glaucoma

• Glaucoma is a state of elevated pressure within the eye that occurs when the scleral then venous sinus is obstructed and aqueous humour is not reabsorbed as fast as it is secreted. Pressure in the anterior and posterior chambers drives the lens back and puts pressure on the vitreous body. The vitreous body presses the retina against the choroid and compresses the blood vessels that nurish the retina. Without a good blood supply, the retinal cells die and the optic nerve atrophy, producing blindness.
• Symptoms often go unnoticed until the damage is irreversible.
• In late stages, they include, dimness of vision, reduced visual field , and coloured halos around artificial light.
• Glaucoma can be halted with drugs or surgery, but lost vision cannot be restored.

Other Conditions

Colour Blindness

• Is a colour vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish.
• It is most often of genetic nature and can be found in about 8% of males and 0.4% of females.  These condition is linked to the X chromosome and tend to be passed from a mother to her son.  
• Colour blindness may also occur due to eye, nerve, or brain damage, or due to exposure to certain chemicals.
• The normal human retina contains two kinds of light cells: the rod cells (active in low light) and the cone cells (active in normal daylight).There are three kinds of cones, each containing a different pigment. The cones are activated when the pigments absorb light. The absorption spectra of the cones differ one is maximally sensitive to short wavelengths, one to medium wavelengths, the third to long wavelengths (their peak sensitivities are in the blue, yellowish-green, and yellow regions of the spectrum, respectively). The absorption spectra of all three systems cover much of the visible spectrum, so it is not entirely accurate to refer to them as "blue", "green" and "red" receptors, especially because the "red" receptor actually has its peak sensitivity in the yellow.
• The sensitivity of normal color vision actually depends on the overlap between the absorption spectra of the three systems: different colors are recognized when the different types of cone are stimulated to different extents.
• There is no treatment or cure for colour blindness. Those with mild colour deficiencies learn to associate colours with certain objects and are usually able to identify colour as everyone else.  However, they are unable to appreciate colour in the same way as those with normal colour vision.

More information found at https://en.wikipedia.org/wiki/Colour_blindness