5.2.3 The eye

The Eye Is a Sense Organ That Detects Light

1. The eye is a sense organ containing receptors sensitive to light intensity and colour.
2. These receptors are found in the retina at the back of the eye.
3. Light is focused onto the retina, which sends impulses along the optic nerve to the brain.

The Main Structures of the Eye

1. Sclera
   1. The tough white outer layer that protects the eyeball.
2. Cornea
   1. The transparent front of the eye that lets light in and does most of the refraction (bending) of the light.
3. Iris
   1. The coloured ring of muscle that controls the size of the pupil and so the amount of light entering.
4. Lens
   1. A transparent disc that changes shape to fine-focus the light onto the retina.
5. Ciliary muscles and suspensory ligaments
   1. The ciliary muscles are a ring of muscle that changes the lens shape; the suspensory ligaments attach the lens to them.
6. Retina and optic nerve
   1. The retina is the light-sensitive layer of receptor cells; the optic nerve carries the impulses to the brain.

Adapting to Dim and Bright Light: The Pupil Reflex

1. Changing the pupil size is a reflex that protects the retina and helps you see in different light levels.
2. In dim light the radial muscles contract and the circular muscles relax, so the pupil dilates (widens) and more light enters.
3. In bright light the circular muscles contract and the radial muscles relax, so the pupil constricts (narrows) and less light enters, protecting the retina.

Accommodation: Focusing on Near and Distant Objects

Focusing on a near object

1. The ciliary muscles contract.
2. The suspensory ligaments loosen.
3. The lens becomes thicker and more curved, so it refracts light rays strongly.

Focusing on a distant object

1. The ciliary muscles relax.
2. The suspensory ligaments are pulled tight.
3. The lens is pulled thin, so it only slightly refracts the light rays.

Myopia: Short-Sightedness

1. In myopia (short-sightedness), distant objects look blurred but near objects are clear.
2. The light is focused in front of the retina, because the eyeball is too long or the lens is too curved.
3. It is corrected with a concave (diverging) lens, which spreads the light out before it enters the eye so it focuses on the retina.

Hyperopia and New Technologies

1. In hyperopia (long-sightedness), near objects look blurred but distant objects are clear.
2. The light is focused behind the retina, because the eyeball is too short or the lens is too flat.
3. It is corrected with a convex (converging) lens, which brings the light rays together before they enter the eye.
4. New treatments
   1. Hard and soft contact lenses sit on the surface of the eye and are useful for sport.
   2. Laser surgery changes the shape of the cornea to alter how much it refracts light.
   3. A replacement lens can be put into the eye to correct the defect.