REFRACTION

REFRACTION

• When light rays reach an angulated surface of a different material, it causes the light rays to bend. This is called refraction. When light reaches a convex lens, the light rays bend toward the center

• When light rays reach a concave lens, the light rays bend away from the center:

The eye has multiple angulated surfaces that cause light to bend. These are: The interface between the air and the front of the cornea The interface between the back of the cornea and the aqueous humor The interface between the aqueous humor and the front of the lens The interface between the back of the lens and the vitreous humor When everything is working correctly, light makes it through these four interfaces and arrives at the retina in perfect focus.

Causes of Refractive ErrorsThe eye's ability to refract or focus light sharply on the retina primarily is based on three eye anatomy features: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye.Eye Length: If the eye is too long, light is focused before it reaches the retina, causing nearsightedness. If the eye is too short, light is not focused by the time it reaches the retina. This causes farsightedness or hyperopia.Curvature of the Cornea: If the cornea is not perfectly spherical, then the image is refracted or focused irregularly to create a condition called astigmatism. A person can be nearsighted or farsighted with or without astigmatism.Curvature of the Lens: If the lens is too steeply curved in relation to the length of the eye and the curvature of the cornea, this causes nearsightedness. If the lens is too flat, the result is farsightedness.

Detection of Refractive Errors

• Eye doctor determines the type and degree of refractive error by performing a test called a refraction.

• This can be be done with a computerized instrument (automated refraction) or with a mechanical instrument called a phoropter (manual refraction).

• The refraction may reveal that patient have more than one type of refractive error. For example, blurred vision may be due to both nearsighted and astigmatism.

Myopia

Myopia • A myopic, or nearsighted person has difficulty

seeing distant objects clearly, such as road signs, blackboards in a classroom. Myopia varies in its severity. Extremely nearsighted people may only have clear vision up to a few inches from their eyes; mildly nearsighted people may not even be aware of the disorder.

• Myopia develops during childhood. The child’s eyeball grows too long, which interferes with the light-focusing mechanism. Rays of light from distant objects focus in front of the retina rather than on the retina, resulting in blurry vision. This can also occur if the cornea is curved too acutely.

Myopia

• Incidence and Prevalence • Myopia is fairly common worldwide.

Approximately 25% of the world population is myopic to some degree, affecting men and women equally. There is greater prevalence of this disorder among children from higher socioeconomic groups, although the reasons for this remain unclear. Incidence is much higher in certain Asian countries. Taiwan experiences the world’s highest rate. In some nations, such as Singapore, the incidence has rapidly increased over the past decade.

Myopia • Risk Factors • While the exact cause of myopia remains unknown, it is believed that an

interaction of heredity and environment may play a role. It tends to run in families. When both parents are nearsighted, their children have a greater than average chance of developing myopia. Environmental factors such as reading in dim light or doing excessive amounts of close work may contribute to myopia.

• Signs and Symptoms • Symptoms of myopia usually are first noticed in childhood. Myopic children

may hold their books very close to their face or be unable to read the blackboard at school. They may squint and complain of headaches and eyestrain. These signs often alert a teacher or parent that the child is having vision problems and that an eye exam is needed.

• Diagnosis • A diagnosis of myopia can be made only after a basic eye examination

performed by an ophthalmologist or an optometrist. The initial diagnosis of myopia is usually made by having the patient read letters from an eye chart. Other tests are performed to determine the degree of myopia.

Myopia

• Treatment • Treatment for nearsightedness is most

commonly prescription eyeglasses or contact lenses. Wearing the glasses or contact lenses allows the patient to experience normal or at least dramatically improved, vision.

• Myopia often worsens progressively during adolescence, and the eyeglass or contact lens prescription may need to be adjusted periodically. However, myopia tends to stabilize once the patient is in their twenties.

Myopia

• A number of surgical procedures are used to correct refractive errors, including radial keratotomy, photorefractive keratotomy, clear lens replacement therapy, and LASIK. These procedures reshape the central area of the cornea. In many cases, correction makes eyeglasses unnecessary. Not every patient is a good candidate for surgical correction. Surgery is not recommended for people under the age of 18 because their eyes are still growing

Myopia

• Prevention • There is no way to prevent myopia. Avoiding eyestrain,

reading and studying in a well-lit room, and taking breaks from working in front of a computer screen or doing close work may help. Eye health can be supported by the following:

• Eating a healthy diet that is rich in vitamins A and C • Protecting the eyes from excessive amounts of

ultraviolet light by wearing sunglasses when outside • Drinking adequate fluids to prevent eye dryness • Protecting the eyes when working with hazardous or

caustic substances • Having regular eye exams, especially if at high risk for

eye disease (e.g., diabetic)

Hyperopia

Hyperopia • Hyperopia, or farsightedness, is a disorder where distant

objects are usually seen clearly, but close ones do not come into proper focus and are blurred. This occurs when the eyeball is too short or the cornea is too flat, and light rays entering the eye focus behind the retina rather than directly on it.

• Hyperopia is often present at birth, but sometimes, vision normalizes as the eyeball lengthens during the growth process. Young adults and children with mild to moderate cases of farsightedness often see close objects clearly because the lens is able to adjust or change its shape. This process is called accommodation. As a person ages, the ability to accommodate often lessens and eyeglasses or contact lenses may be needed.

Hyperopia

• Farsightedness is a risk factor for closed-angle glaucoma. Therefore, patients with hyperopia should discuss glaucoma testing with their eye care practitioner.

• Incidence and Prevalence Approximately one-quarter of the world population is hyperopic, and incidence increases with age. At least half of all persons over the age of 65 have some degree of hyperopia. Men and women appear to be affected equally.

Hyperopia

• Risk Factors • Because it is often present at birth, there may be

a hereditary risk factor. Age is also a risk factor for hyperopia.

• Causes • Hyperopia results when the eyeball is too short

or the cornea is too flat and the lens is unable to adjust its shape to see close objects clearly (the process of accommodation).

Hyperopia

• Signs and Symptoms • Most children and young adults with mild to moderate

hyperopia do not experience symptoms because of the lens’s ability to accommodate. Older persons and those with more severe hyperopia may experience these symptoms:

• Achy feeling in eyes • Blurred vision of close objects • Eye strain • Headache • Diagnosis • A diagnosis of hyperopia can be made only after a basic

eye examination performed by an ophthalmologist or an optometrist.

Hyperopia • Treatment • Treatment for farsightedness is prescription eyeglasses

or contact lenses. Wearing the glasses or contact lenses allows the patient to experience normal, or dramatically improved, vision. Many farsighted people wear their glasses only when doing close work, such as reading or sewing.

• The surgical procedures are used to correct refractive errors, including photorefractive keratotomy, LASIK, and clear lens replacement therapy. Some of these procedures are appropriate for hyperopia, and in many cases, surgical correction can make eyeglasses unnecessary. Not every patient is a good candidate for surgical correction.

• Prevention • There is no way to prevent hyperopia.