CSEC - The eye and how it works
How the eyes adapt to different light intensities
Too much light would damage the receptor cells in the retina. Too little light wouldn't allow them to work properly.
The iris contains a pigment that gives our eyes their nice colour, this protects the retina from getting too much light. A darker iris colour gives more protection. But that isn't flexible enough to cope with constant changes in light intensity.
Instead, there is yet another reflex!
The iris contains two sets of muscles
One muscle is radial, arranged round the pupil like spokes in a wheel.
The other is circular, arranged concentric rings around the pupil.
When there is too much light, the pupil is closed down by contracting the circular muscle.
When there isn't enough light, the radial muscles in the iris contract, pulling the pupil wider. This lets more light through to the retina.
Each iris has both circular and radial muscles in it at the same time. Have a look in a mirror, or at your friend's eye - much more fun!
Adapting to different objects - Accommodation
The eye also has to adapt to be able to focus a clear image of an object no matter how far away it is from the eye. Again, this is under the control of the nervous system.
When the eye looks at an object that is far away the ciliary muscle pulls on the suspensory ligaments. These pull on the lens and make it flatter (less convex). This brings the rays of light from the object into crisp focus on the retina.
But with an object closer to the eye, the lens needs to be more convex (fatter). To do this, the ciliary muscles relax to allow the rubbery lens to return to its naturally rounder shape.
The brain's careful control of the ciliary muscles allow it to adjust the convexity of the lens to give a perfectly focussed image on the retina.
This retinal image is not the same as the object that is being looked at. The image is inverted.
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