We see the world around us because of the way our eyes bend (refract) light. Refractive errors prevent the eye from properly focusing light, causing blurred vision. The main refractive errors are nearsightedness, farsightedness and astigmatism.
Refractive errors can usually be "corrected" with glasses or contact lenses, or they can be permanently treated with vision correction surgery like LASIK. To find out more about refractive errors, let's look a little closer at how the eye works.
How the Eye Sees
The process of vision begins when light rays that reflect off objects and travel through the eye's optical system are refracted and focused into a point of sharp focus. For good vision, this focus point needs to be on the retina. The retina is the tissue that lines the inside of the back of the eye, where light-sensitive cells (photoreceptors) capture images in much the same way that film in a camera does when exposed to light. These images then are transmitted through the eye's optic nerve to the brain for interpretation.
Causes of Refractive Errors
The eye's ability to focus light sharply on the retina depends on three main features of the eye: 1) the overall length of the eye, 2) the curvature of the cornea and 3) the curvature of the lens inside the eye. Both genetics and environmental factors (e.g. quantity of close work like reading, screen time etc) can affect these physical features of 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.