Myopia Also konwn as 'nearsightedness', a person suffering from this disease is only able to see objects near the eye clearly. Most of the prople who wear spectalcles are patients of myopia. The disease can be caused due to two reasons. First, the lens might be too thick and fails to focus light rays coming from distant objects onto the retina. Second reason could be a smaller eye ball. Since image is formed behind the retina in the eye of a myopic person, a divergent lens is required to enable him to see distant objects clearly.

The power of lens required for a myopic person who can see an object at a maximum distance of x, is given by P = -(1 / x)

Hypermetropia Also konwn as 'hyperopia' or 'farsightedness', a person suffering from this disease is only able to see objects far from the eye clearly. This disease manily affects old people. It can be due to two reasons. First, the lens is too thin and fails to focus light rays coming from close objects onto the retina. Second reason could be a larger eye ball. Since image is formed in front of the retina in the eye of a hypermetropic person, a convergent lens is required to enable him to see close objects clearly.

The power of lens required for a hypermetropic person who can see an object at a minimum distance of y, is given by P = -(1 / y)

Glaucoma It is a symptomatic condition, and can lead to raised intra-ocular pressure, visual field loss, enlargement of the blind spot and changes in the appearance of the optic nerve head. a lot of damage is done by the time these symptoms are actually noticed by the patient.

Chorioditis Choroiditis is an inflammation of the choroid, which is a vascular rich layer located between the sclera and the retina.

Astigmatism This defect arises due to the develoment of different curvatures along different planes in the eye lens. A person suffering from astigmatism cannot see all the directions equally well, but a particular direction in the plane perpendicular to the lign of sight is most visible. There are many ways with which a person can detect this disease, many of which are 'do-it-yourself' technices. A person suffering from this disease wears glasses with different curvatures in different planes, which are known as cylindrical glasses.

Presbypia It is a reduction in the focusing power of the eye with age, and results in a decrease in clarity of vision at near distances (but does not affect distance vision). When a person can no longer read a newspaper at arm's length, and buys reading spectacles, he can be considered to be presbyopic.

Blindness This refers to the loss of the power of sight. Depending on his reduced power, a person can be calassified as 'stone-blind' or 'gravel-blind' (Shakespearen terms referring to the dergree of blindness). Some of the factors that might lead to blindness are: Physical damage: This includes a direct damage to the retina or the lens, in cases that include accidents involving sharp things, metal instruments etc. that might cause a major rupture. Retinal detachment: This might happen due to the separation of the neural retina from the pigment epithelium, which provides nutrients to the photoreceptors. Due to this loss of nutrients, there is a loss of function in the photoreceptors. Blindness however, is caused in extreme cases only. Minor detachments only cause a blurred vision.

Keratoconus It is usually a recessive inherited condition apparent between 10-25yrs. It leads to a thinning of the corneal apex, and a bulging of the tissue which incereases myopia of the afected eye.

Herpes Zoster This disease affects the eye by encroaching from a skin lesion. It might even start on the eye. Leads to white pustules, similar to cold sores in the mouth. It can be treated by early antiviral treatment.

Retinitis Pigmentosa It is a degeneration of the retinal pigment epithelium, which leads ro the loss of granules from the epithelium layer and their deposition in clumps in the retina.

Migraines: This disease affects all age groups and about 10% of the world population. There is a constriction of blood vessel(s) followed by a dilation. Aura, when present, accompanies vasoconstriction and headache, if present, accompanies vasodilation. Migraines can be caused due to certain foods (which contain caffiene citrus juices, alcohol, nitritate and nitrites), hormonal changes (like puberty, pregnancy, and menopause), fatigue (emotional or physical), bright lights, loud noises, and trauma. Migraines are heriditary, and can be passed down from generation to generation.

Migraines can be classified into common migraine (about 80% of the total patients, is accopanied by headache and mood changes), classical migraine (about 10% of the total patients, is accompanied by aura, headache and nausea/vomiting), complicated migraine and cluster headaches (with, nasal stiffness, facial pain and lacrimation).

A complicated migraine can be one of the following types: Cerebral: severe headache and long-lasting focal neurologic signs, can lead to permanent visual field effects. Ophthalmoplegic: affects patients under 30yrs., severe headache with paretic ocular muscles, same symptoms as aneurysm or tumour. Retinal/Ocular: affects patients under 40yrs., loss of visual field in one eye,extent of loss determinig whther it is retinal or ocular. Basilar: symptoms same as vertebrobasilar attacks, bilateral blurred vision, vertigo, ataxia, nausea, loss of balance and speech difficulties. Other types: chest pains, vomiting, neurologic symptoms and other sysmptom complexes, sometimes known as migraine equivalents.

Cataract Cataracts are opacifications of the crystalline lens of the eye, causing a loss of transparency. They can be caused due to accumulation of water or denaturation of the lens protein, because of diabetes, eye trauma or old-age. Predominant symptoms of cataract are increasing loss of vision and associated fluctuations in vision. A cataract can be easily removed by surgery, which can be two of the following types: Phacoemulsification (PE). Nucleus expression cataract extraction (ECCE). The first type of surgery is mostly preferred unless in a case where ECCE can give better results depending upon the type of cataract.

Conjunctivitis This is an inflammation of a mucous membrane, and can lead to red eye, thickening of the conjunctival tissue and some discharge of mucous or mucous and inflammatory cells. It can be caused because of: Bacterial infection: Due to bactrea like the common staphylococcus and diplococcus pneumoniae. It is in the form of a "smarting" and grittiness, moderate photophobia. Discharge can cause eyelids to stick together or have a "crusty" appearence. Viral infection: Is caused by adenoviruses, often involved in upper respiratory tract infections. Causes inflammation of the membrane at the back of the eyelid. Allergic reactions: results from hypersensitivity and exogenous antigens. Causes profuse watering due to hay fever and chronic inflammation.

Defective Colour Vision: This disease reduces the ability of a person to distinguish between different colours. Defects in colour vision, often incorrectly referred to as "colour blindness" fall into two main categories: 1). Congenital Colour Vision Defects They can be either one of the following three types: (a) Achromatic (Monochromatic) Colour Vision: includes no colour vision, only light - dark discrimination, lack of retinal function and higher centre defects. (b) Dichromatic Colour Vision: includes colour distinctions of two kinds, can be either protanopia, deutranopia, tritanopia or tetartanopia. (c) Trichromatic Colour Vision: can be either protan, deutran or tritan. These type of colour vision defects are heriditary. 2). Acquired Colour Vision Defects They can be due to lesions of the macula, optic nerve or visual cortex. Changes in the optical media or toxic effects of chemicals can alter colour perception. They are usually asymmetrical in the two eyes.

Strabismus (Squint Eyed) A squint or strabismus is a failure of the two eyes to look at the same object thereby preventing binocular vision. Interruption to the motor, sensory or central components can lead to sensory or central defect. Causes of many squints are not fully understood although the in major cases, the disease is either due to a hereditary factor or a responsible defect. Certain factors causing suints include ocular abnormalities, paresis of one of the eye's muscle, brain damage or large refractive errors.