MadSci Network: Anatomy
Query:

Re: what is acommodation using eyes and light?

Date: Wed Apr 11 12:58:14 2001
Posted By: Benoit A. Bacon, Post-doc/Fellow, Psychology, University of Glasgow
Area of science: Anatomy
ID: 974840231.An
Message:

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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