Accommodation of the Eye

Now that we know that the photoreceptors are on our retina, the question is: How can the image fall onto the retina? ----- by focusing. To understand focusing, you'll need a brush-up on physical optics first.

1) What is accommodation ?

Accommodation is to change the focal length of the lens by changing the curvature of the eyelens.

2) Why does our eye need accommodation?

Normally, when our ciliary muscles are relaxed, parallel rays form distant objects will converge onto the retina.

If our eye is maintained at the above state, and a near object is put before it, light rays will converge behind the retina. As the sharp image is behind the retina, our brain can only detect a blurry image.

To bring the image into focus, our eye does accommodation.

3) How does our eye accommodate?

Remember that the cornea provides 2/3 of the refractive power and the lens only provide 1/3 ? However, our eye changes the curvature of the lens, rather than the cornea. The curvature of the cornea cannot be changed. .

Changing the thickness of the lens means changing the focal length of the lens.

Our eye lens is made of an elastic lens capsule and filled with compressible? lens substance. It is suspended by Zonular ligaments to the ciliary muscles.

In the normal resting state:

• our ciliary muscle is relaxed
• the elastic lens tends to become thick
• aqueous & vitreous humour push outward on the sclerotic coat
• ligaments become taut / tensed
• lens pulled into a thin shape
• short focal length

This is what happens when a near object is brought to our eye:

1. sphincterlike action of circular muscle fibres + contraction of longitudinal muscle fibres
2. contraction of ciliary muscle
3. distance between edges of ciliary body decreases
4. relaxation of suspensory ligament
5. lens becomes thicker
6. focal length shortens
7. light rays converge earlier; image formed on retina

When we grow old, our lens will turn hard. Our accommodation ability will decrease and it will get more and more difficult to focus. This defect is called presbyopia.

Bony fish do not change the focal length of their lens. Instead, they elongate their eyeballs so that the image falls right on the retina.

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