MAVO Year 4 · Senses · Section 11.4

The iris and lens

The pupil reflex

The pupil reflex adjusts the amount of light entering the eye. In bright light, the circular muscles of the iris contract, narrowing the pupil. In dim light these muscles relax while the radial muscles contract, widening the pupil. This automatic adjustment helps vision under different light conditions and limits incoming light, although it does not make looking at very bright sources safe.

Image formation

The cornea and lens refract light, producing a smaller, inverted image of most objects on the retina. The brain interprets the pattern of retinal signals together with experience and other information, allowing us to perceive a stable world. It does not literally turn a small picture upright inside the head.

Focusing on distant and nearby objects

The ciliary muscle, around the lens, is separate from the pupil-controlling muscles in the iris.

• Distant objects: the ciliary muscle relaxes, tension in the suspensory ligaments increases and the lens becomes flatter. It refracts light less strongly.
• Nearby objects: the ciliary muscle contracts, tension in the ligaments decreases and the elastic lens becomes more rounded. It refracts light more strongly.

Accommodation

Changing the lens's focusing power is called accommodation. It allows clear vision at different distances. With age, the lens becomes less flexible and cannot become sufficiently rounded for nearby objects. This is presbyopia, a common reason for needing reading glasses.