Refraction of the Eye and Refractive Errors
What is Refraction?
Refraction is a term used in optical physics and describes “the bending of light rays at the interface between two different transparent media”. It is the phenomenon which makes image formation possible by the eye as well as by cameras and other systems of lenses.
Light rays enter the eye through our cornea, pupil and lens in that order. The cornea is the clear, round dome covering the iris and pupil. The light rays then pass through the lens, then through the vitreous and are finally focused on the retina – the light sensitive tissue lining the back of our eye. By helping to focus light as it enters the eye, the cornea and the lens both play important roles in giving us clear vision. The cornea is responsible for 70% of the eye’s focusing power and the lens accounts for 30%. Light from an object is passed through the eye and the image that is formed on the retina is inverted (upside down). The image is interpreted the right way up by our brain which is connected to the eye via the optic nerve. Click image below to enlarge.
You can read our previous post on the retina here: http://sosdoctors.com.au/back-to-basics-the-retina/
In Ophthalmology, the term refraction refers to the various testing procedures employed to measure the refractive errors of the eye to provide the appropriate correction.
What is Refractive Error?
Refractive error is by far the most common cause of poor vision. Fortunately it is generally the easiest to treat. If the cornea is not as smooth, clear or round as it should be, or the eyeball is too long or too short, light rays will refract (or bend) at odd angles, leading to blurry or distorted vision. This inability to achieve sharp focus is called refractive error. [https://www.aao.org]
Refractive errors include Myopia or near-sightedness; Hyperopia or far-sightedness; or Astigmatism. Similarly the eye will have focusing problems if the naturally clear lens is cloudy due to a cataract (see http://sosdoctors.com.au/back-to-basics-all-about-cataracts/ ); or if the lens is not as flexible as it should be which is called Presbyopia. We will be looking at each of these types of refractive errors along with presbyopia throughout the week.
We measure refraction in Dioptres (D), which describes the power that a structure needs to focus parallel rays of light to bring them to a point on the retina. Stay tuned to find out more about different types of refractive errors and how we treat them as we continue our back to basics series this week!
References: https://www.aao.org; http://patient.info; http://www.GCSE/physics