The eye works like a camera that focuses light onto a layer of nerve cells at the back. The brain then turns those signals into the picture we see.
Focusing light
Light passes through the cornea, the pupil and the lens, which bend it to a sharp point on the retina. The lens changes shape to focus on near objects and stiffens with age, which is why reading glasses become common after 40.
The retina
Rods work in dim light and see shades of gray; cones work in bright light and see color and detail. The macula, and its center the fovea, is packed with cones and gives sharp central vision.
The visual pathway
Each optic nerve carries signals back from one eye. At the optic chiasm the fibers from the inner (nasal) half of each retina cross, so each side of the brain sees the opposite half of the visual field, and the signals end in the occipital lobe.
Pupil control
Light in either eye makes both pupils constrict through the oculomotor nerve (cranial nerve III), a parasympathetic pathway. Sympathetic nerves dilate the pupils in the dark or under stress.
At the bedside
Pupil size, symmetry and light reaction are quick checks of the brainstem and the third nerve. A visual field defect can show where along the pathway a stroke or tumor sits.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).