Light into the eye
Seeing begins with light. Most of the objects around you do not make their own light; they reflect it, and some of that reflected light enters your eyes. The eye’s job is to focus that light into a sharp image on a light-sensitive layer at the back, and then convert it into signals the brain can use.
The path of light
Light passes through several structures in order:
- Cornea. The clear, curved front surface of the eye. It does most of the eye’s focusing, because it bends light more than any other part.
- Pupil and iris. The iris is the colored ring of muscle, and the pupil is the opening in its center. In bright light the pupil narrows, and in dim light it widens, controlling how much light gets in.
- Lens. A clear, flexible structure behind the iris that fine-tunes the focus. Small muscles change its shape: it becomes rounder for close objects and flatter for distant ones. This adjustment is called accommodation.
- Vitreous humor. A clear gel that fills the main chamber of the eye and lets light pass through to the back.
Bending of light by clear curved surfaces is an example of refraction, one of the ideas covered in the broader science of light.
The retina
The retina lines the back of the eye. The image formed on it is inverted, meaning upside down and left-right reversed, much like the image in a simple camera. The brain handles this without any conscious effort, so the world does not appear upside down.
The retina contains two main kinds of light-sensing cells.
- Rods are very sensitive and work in dim light, but they do not distinguish color. They are mostly responsible for night vision and are concentrated away from the center of the retina.
- Cones need brighter light and provide color vision and fine detail. Most people have three types of cones, tuned broadly to shorter (bluish), medium (greenish) and longer (reddish) wavelengths. The brain compares their relative responses to produce the many colors we perceive.
Cones are packed most densely in a small central region called the fovea, which provides the sharpest vision. This is why you turn your eyes to look straight at something you want to see clearly.
From light to signal
Rods and cones contain pigment molecules that change shape when they absorb light. This triggers a chain of chemical reactions inside the cell that changes its electrical state and its release of chemical messengers. Nearby retinal cells begin processing the information right away, comparing signals and enhancing contrast, before sending the result along the axons of retinal ganglion cells. These axons bundle together to form the optic nerve. How signals travel along nerve fibers is explained in the article on neuron signals.
The blind spot
Where the optic nerve leaves the eye there are no rods or cones, so that spot cannot detect light. This is the blind spot. We rarely notice it, because the brain fills in the missing patch using information from the surroundings and from the other eye.
The brain builds the image
The signals travel through a relay station in the brain, the thalamus, to the visual cortex at the back of the brain. Information from the right half of each eye’s field of view ends up in the left side of the brain, and vice versa. The visual cortex and nearby regions analyze edges, movement, color, depth and shape, and other areas help recognize objects and faces.
Depth perception relies partly on the slightly different views from the two eyes, and partly on cues such as size, overlap and shadow, which work even with one eye.
Vision is not a simple recording. The brain makes assumptions, and your expectations and past experience influence what you perceive. Optical illusions reveal these assumptions. This is one of the many brain facts that connect to larger questions about the human mind.
Common focusing problems
When the eye’s focusing power does not match its length, images do not land exactly on the retina.
- In nearsightedness (myopia), distant objects look blurry, often because the eye is slightly long.
- In farsightedness (hyperopia), close objects can be hard to focus on, often because the eye is slightly short.
- In astigmatism, an uneven curve of the cornea or lens blurs vision at various distances.
- With age, the lens becomes less flexible, so close focusing gets harder, a change called presbyopia that most people notice in middle age.
Glasses, contact lenses and some surgical procedures can correct these focusing problems, which are common.
Color vision differences
Some people have cones that respond differently, which is usually described as color blindness. It is more common in males and often inherited, and it commonly involves difficulty telling certain reds and greens apart. Most people with it can see colors, just not in exactly the same way.
A note on eye health
This article is general educational information and not medical advice. Sudden changes in vision, eye pain or flashes of light are reasons to contact a qualified eye care or medical professional promptly, and routine eye checks are widely recommended because individual cases differ.