MadSci Network: Anatomy |
Dear Kathryn, Thank you for your very interesting question about accommodation, which is indeed the process that allow our eyes to be always in focus and, as you say, to "see all objects normally wherever wee look". As you probably know, light enters our eyes and passes through a series of structures (cornea, aqueous humour, crystalline lens and vitreous humour) before reaching the retina (see the picture). We only see objects clearly when their image (the light they reflect into the eye) falls directly upon the retina. If objects are far enough (several meters), their image will automatically fall on the retina. However, when objects become closer, the eye must change its focus to place their image on the retina. This change of focus, which is called accommodation, is achieved by changing the shape of the crystalline lens. To focus on near objects, the ciliary muscles tighten, which relaxes the tension on the zonular fibres. As a result, the lens gets thicker and light is more refracted, so as to fall on the retina. Then when you look at a farther object, the ciliary muscle relaxes, the zonular fibres tighten and the lens becomes thinner, refracting the light less and once again placing the image of the object on the retina! This ability of the eye to accommodate is automatic. Furthermore, it occurs instantaneously. Focus on a far away object and quickly turn your attention to a nearby object; observe that there is no noticeable delay in the ability of the eye to bring the nearby object into focus. Accommodation is a remarkable feat! I hope this helps Cheers Benoit
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