Refractive error is a common eye problem wherein your eyes are not focussing the light properly. In the normal eye, the light rays entering are focussed directly on the retina – light-sensitive tissue or layer lining the back of our eye. If the eye is not focusing the light rays on the retina, the image will be a blur. And this condition is called a refractive error.
First, let us see how the eye works?
It is easy to understand the eyes when we compare them with the camera. For better understanding, we separate the whole process and explain in step by step manner.
- Firstly the light rays enter through cornea – clear transparent front covering of the eye. Cornea’s refractive power is so powerful that it bends light rays such a way that it passes through the pupil- the opening in the iris. The Iris acts same as a shutter in the camera. Added Iris has the ability to dilate and shrinks depending on how much light entering the eye.
- After passing through the pupil, the light rays enter the crystalline lens as on the camera. The beauty of this lens is that it can change its curvature in order to focus light precisely on the retina.
- Light rays after refracting from the crystalline lens pass through the clear transparent fluid or gel called vitreous. The Vitreous fills the globe of the eye and its main function is to maintain the spherical shape of the eyes.
- Finally, the light rays are focussed on the retina. Retina acts as a film in a camera. It has a light-sensitive receptor called rods and cones which captures all the light rays and convert them into photo-sensitive signals and sends to the visual cortex in the brain, then the brain does all miracles. Before the brain fuses the image into the single it actually receives two light signals from two separate eyes. The brain processes the image into single and thus we see the beauty of the world.
Common Refractive Error
Myopia (distant object appears to be blurred and the near object clear)
In myopia, the length of the eyeball we call it as an ‘axial length’ is longer than the normal so the light rays fall in front of the retina but not on the retina. Or it can be because of the steeper cornea, the dioptric strength of the cornea increases and so that light is bent more than what is required, ultimately the light rays finally focussed in front of the retina.
Hyperopia (distant object appears to be clearer but the near objects blur)
In Hyperopia, like myopia but straightly opposite, the axial length is shorter so that light rays fall behind the retina. The second reason could be because of the flat cornea, the dioptric power of the cornea decreases, that the light falls behind the retina.
In Astigmatism, light rays entering the eyes are not focussing on a single common point rather one meridian fall in front and another meridian falls behind. Most of the times this occurs because of the irregular cornea but sometimes it can be also due to the curvature of the lens. A normal cornea is round, curved like basketball but in astigmatism, cornea curves more in one direction than other like a football. This makes dioptric power difference between the two meridians. This causes the image to be blurred and stretched out.
Presbyopia (normal ageing condition)
Presbyopia is a normal ageing condition which occurs mostly after 38 years of age. This is because of the hardening of the crystalline lens causing the eye to focus light rays behind the retina during near activities like reading new paper.