What is the retina?
General information only. This article explains the basics and is not medical advice. For anything about your own health, talk to a doctor or other qualified professional.
Quick answer
The retina is the thin, light-sensitive tissue lining the back of the eye. It contains millions of cells called rods and cones, which turn light into electrical signals. The optic nerve carries those signals to the brain, which turns them into the images we see. It works a bit like the sensor in a camera.
Rods and cones
| Cell | Number (approx.) | Best for |
|---|---|---|
| Rods | About 120 million per eye | Dim light, night vision, movement; no color |
| Cones | About 6 million per eye | Color and fine detail, in bright light |
Humans have three types of cones, sensitive to roughly red, green and blue light. The brain combines their signals to see millions of colors. If one type is missing or works differently, a person may have color blindness.
The fovea: sharpest vision
Near the center of the retina is a small pit called the fovea, packed almost entirely with cones. When you look straight at something to read or recognize a face, you’re aiming its image at your fovea.
The blind spot
Where the optic nerve leaves the eye, there are no rods or cones, creating a blind spot in each eye. You don’t notice it because your brain fills in the gap and your other eye covers it.
Try it: Draw an X and a dot about 8 cm apart. Close your left eye, stare at the X with your right eye, and slowly move the page toward you. At one point, the dot disappears.
An upside-down image
The eye’s lens projects an upside-down image onto the retina, as in a camera. The brain flips it so we see the world the right way up. Light bending through the lens is explained in what refraction is.
How light reaches the retina
Light passes through the cornea, then the pupil, whose size is set by the iris; see which part of the eye controls light. Then it goes through the lens and the clear gel inside the eye before reaching the retina.
Retina facts
- The retina is about as thin as a sheet of paper, roughly 0.2–0.5 mm.
- It’s actually an extension of the brain, formed from the same tissue during development.
- Eye doctors can see retinal blood vessels directly, which helps them spot signs of conditions such as diabetes and high blood pressure.
For the animal with the biggest eyes for its size, see which primate has the biggest eyes.
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