The eye works in a similar way as a camera. To get a clear picture, the lens of the camera focuses the rays of light on the camera film. In a similar way, the light rays get focused on the Retina (the innermost layer of the eye) with the help of Cornea (the anterior surface of the eye) and lens of the eye. The inability of the eye to accurately focus these rays of light coming from a distance, onto the retina is called Refractive Error. This condition may be either because the eye is too short or long in length, or because the refractive power of the cornea or lens is not optimum.
There are three types of refractive errors:
Myopia (near-sight): The light rays are focused in front of the retina, either because the eye is too long or the power of the cornea and lens is more than required. Distant objects are blurred but the near objects are seen clearly. Minus glasses are needed to correct this condition.
Hypermetropia (long-sight): The light rays are focused behind the retina, either because the eye is too short or the power of the cornea and lens is less than required. The person may need to strain the eyes to see objects clearly. Plus glasses are needed to correct this condition.
Astigmatism: the light rays do not get focused evenly to a point, usually due to the cornea of the eye being more curved in one direction than the other. Cylinder lenses (with axis) are needed.