Structure and Function of the Eye

The eye does more than admit light. It detects patterns of light from the surroundings and converts information about them into electrical signals that can be interpreted by the brain. This conversion from a stimulus into a nervous signal is called transduction.

For this to work, light must first be directed onto the light-sensitive retina. The main route is:

cornea → pupil → lens → retina

The cornea and lens focus the light, the iris controls how much enters, and rod and cone cells in the retina detect it.

The cornea begins to focus the light

The cornea is the transparent, curved front surface of the eye. Light changes direction, or refracts, as it enters through the cornea. This begins the process of bringing rays from each point on an object together so that a focused image can form on the retina.

The cornea has a fixed curved shape, so its focusing effect does not alter in the way that the lens does. Its transparency allows light to pass into the eye.

The iris controls light entry

Behind the cornea is the iris, the coloured ring of muscle. The hole in its centre is the pupil. Light passes through this opening; the pupil is not itself a tissue or muscle.

The iris changes the pupil's diameter according to light intensity:

• in bright light, muscles in the iris make the pupil smaller, reducing the amount of light entering the eye;
• in dim light, the iris makes the pupil larger, allowing more light to enter.

This regulation helps the eye work over a range of light intensities. It also limits excessive light reaching the retina in bright conditions.

The lens produces a sharp focus

After passing through the pupil, light reaches the transparent, convex lens. Like the cornea, the lens refracts light. Its particular role is to fine-tune the focus so that light from the viewed object forms a sharp image on the retina.

The lens is flexible. Muscles associated with it can make it flatter or allow it to become thicker, changing how strongly it refracts light. This refocusing for objects at different distances is called accommodation:

• a thicker, more strongly curved lens refracts light more for a nearby object;
• a flatter lens refracts light less for a distant object.

The cornea therefore begins the refraction, while the adjustable lens increases or decreases the convergence needed to bring the image into precise focus on the retina.

The retina detects the focused light

The retina is the light-sensitive layer lining the inner surface at the back of the eye. It contains two types of photoreceptor: rod cells and cone cells. When light stimulates these cells, they transduce the light into electrical signals. Information then passes through retinal nerve cells and leaves the eye in the optic nerve on its way to the brain.

A focused image on the retina is not yet a conscious picture. The brain processes the pattern of nervous signals so that features such as colour, brightness, form and movement can be perceived.

Rod cells: sensitivity in dim light

Rod cells are highly sensitive to light, so they are particularly useful for vision in dim conditions and at night. They do not distinguish colours; their signals provide information about brightness in shades of grey.

Rods are especially important away from the centre of the retina. This helps explain why a faint star can sometimes be seen more clearly by looking slightly to one side of it rather than directly at it: its image then falls on a region with more rods.

Cone cells: colour and fine detail

Cone cells work best in brighter light. Different types respond most strongly to different ranges of wavelength, and the brain compares their activity to produce colour vision.

Cones are concentrated near the centre of the retina, especially in the fovea. This region gives sharp, detailed vision. When you look directly at fine print or a person's face, the eyes position the image on this cone-rich area.

Together, rods and cones allow the eye to operate in very different conditions: rods prioritise sensitivity when light is scarce, while cones provide colour and fine detail when enough light is available.