In simplest terms, refraction is the bending of light rays. Refraction is important in the eye to ensure that light rays are focused onto the light-sensitive back of the eye, the retina.
What is a refractive error?
For good vision, light rays must be focused onto the retina (this is called emmetropia). Frequently, light rays are either refracted too much or too little, and the light is not focused precisely. This is called a refractive error, and results in blurred vision. Most refractive errors can be treated with glasses, contact lenses or surgery.
How common is it to have a refractive error?
Refractive errors are most of the most common eye conditions; in fact, most people have some degree of the condition.
How does refraction occur in the eye?
You may recall from the introduction to the eye that there are 2 structures in the eye responsible for refraction; these are the cornea and the lens.
The cornea, which is the clear, curved covering on the front of the eye, is responsible for most of this refraction (in fact about ¾ of it). The rest of the fraction is done by the lens inside the eye. The lens is attached to a muscular structure called the ciliary body which can alter the shape of the lens to change its focusing power; this allows us to adjust for near and far objects.
Adjusting for near and far objects by changing the shape of the lens is called accommodation.
Note the location of the 2 structures involved in refraction: the cornea and the lens.
What are the different types of refractive errors?
There are 3 different types of refractive errors:
• Myopia: Myopia, or near-sightedness, occurs when the light rays are refracted (bent) too much. Patients with myopia can see well closely, but have blurred vision for far-away objects.
In myopia (near-sightedness), light rays are refracted too much and the image is in focus in front of the retina.
• Hyperopia: Hyperopia, or far-sightedness, occurs when light rays are not refracted (bent) enough. Patients with hyperopia can see well at a distance, but have blurred vision for close objects.
In hyperopia (far-sightedness), light rays are refracted too much and the image is in focus behind the retina.
• Astigmatism: Astigmatism occurs when the cornea is an irregular shape. A useful analogy for astigmatism is to think of the cornea being in the shape of a football instead of a soccer ball. The result is that peripendicular planes of light (i.e. horizontal and vertical) are refracted differently. A blurred image will result.
In astigmatism, horizontal and vertical rays of light are refracted differently.
Who treats refractive errors?
Although ophthalmologists are trained to diagnose and treat refractive errors, it is optometrists who treat most people with refractive errors. An optometrist is trained to determine what type of refractive error exists in a patient, so that correct glasses or contact lenses can be made by an optician.
How is a refractive error treated?
The three most common treatments for a refractive error are glasses, contact lenses and refractive eye surgery. All three of these treatments compensate for the refractive error so that light rays are focused at a point on the retina.
Can you tell me more about refractive eye surgery?
Refractive eye surgery is an increasingly popular way to treat refractive errors. The most common refractive eye surgery is called LASIK. LASIK surgery uses a laser to reshape the cornea and change its refractive power. To read more about LASIK surgery, click here.
