Which are the causes of refractive defects

To obtain a clear vision the different parts of the eye should focus and to refract adequate the rays of light. These rays of light are focused on the retina through the cornea and the crystalline one. The cornea is the "window" transparent in the frontal part of the eye and has to its charge the two thirds of the power of focus of the rays of light. Inside the eye, is the crystalline one the one that provides the third remaining part of this power of focus of the images. The retina, that is the layer that covers the internal wall of the eye grasps the images and transforms them into vision. All the parts of eye should maintain a correct form so that the images can be focused with clarity on the retina by which the form of the eye is an important factor to achieve a clear vision. If the eye is too short or too long the rays of light are not focused appropriate on the retina. If the cornea presents an uneven curvature the vision blurry basket.