The eye

The eye is a sense organ that responds to light.

Structure of the human eye

Light enters through the pupil, and is focused by the cornea and the lens onto the retina. The shape of the lens can be changed by the ciliary muscles so that the image always comes to a sharp focus at the retina.

The iris reflex

The eye needs to be able to control the amount of light entering it. In dim conditions, more light is allowed to enter so that a clear image can be formed on the retina. In bright conditions less light is allowed to enter so that the retina is not damaged.

This adjustment is done by two sets of muscles in the iris: its circular muscles contract to close up the iris, making the pupil smaller - while its radial muscles contract to open up the iris, making the pupil larger. You can see how this reflex action works in the animation.

Accommodation

The ability of the lens to change its shape to focus near and distant objects is called accommodation.

How accomodation works

You can see how this reflex action works in the animation.

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