How You See (Inside the Eye)
Your eye is a living camera. Light bounces off the world, is bent (refracted) by the clear cornea and the flexible lens, then focused onto the retina at the back — where it lands upside-down. There a single photon flips a molecule of rhodopsin and triggers a chemical relay (the phototransduction cascade) that makes a rod or cone hyperpolarize and pass a signal down the optic nerve to the brain, which turns the picture right-side up. Drag the Light slider from starlight to sunlight and watch the pupil narrow, rods hand off to cones, and the photoreceptor voltage swing. Toggle Near/Far to reshape the lens.
What's happening
The Science in Plain Language
1. Light bends twice — refraction. Light travels in straight lines until it crosses from one clear material into another. As rays enter the cornea (the eye's front window) and then the lens, they slow down and bend. Together the cornea and lens do the job of a camera lens: they take rays that are spreading apart and steer them back together. The cornea actually does most of the bending (about two-thirds); the lens fine-tunes the focus.
2. The rays meet at a focus. A healthy eye bends the light just enough that all the rays from one point of the object meet again at a single point on the retina, the light-sensitive lining at the back of the eye. When every point focuses cleanly on the retina, you see a sharp image.
3. The image lands upside-down. Because the rays cross over as they pass through the lens (top rays end up at the bottom, bottom rays end up at the top), the picture projected onto your retina is actually inverted and left-right reversed. Your eye has been doing this your whole life — your brain simply learns to flip it, so the world looks right-side up.
4. The pupil is your aperture. The colored iris is a ring of muscle; the black pupil is just the hole in the middle. In bright light the iris constricts the pupil to about 2 mm to protect the retina and sharpen focus; in darkness it opens to roughly 7–8 mm to gather as much light as possible. That pupillary light reflex happens automatically in a fraction of a second — the same reflex a doctor checks with a penlight.
5. Rods and cones convert light to electricity. The retina holds two kinds of photoreceptor. About 120 million rods are extremely sensitive to dim light and to movement, but see only in shades of grey — they run your night and peripheral vision. About 6 million cones need brighter light and come in three types (roughly red, green, and blue); they give you color and fine detail and are packed most densely at the fovea, the pinpoint at the center of the retina where vision is sharpest. Below about a few lux only rods work (scotopic vision); in bright light rods saturate and cones take over (photopic vision); in between, both contribute (mesopic).
6. Dark adaptation. Step from sunlight into a dark room and at first you see almost nothing. Over the next 20–30 minutes your rods slowly regenerate their pigment and grow dramatically more sensitive — cones adapt within a few minutes, then rods take over and keep improving. That is why night vision "creeps in," and why a single bright flash can wipe it out in an instant. (Making rhodopsin requires vitamin A, which is why a severe deficiency causes night blindness.)
7. Phototransduction — how a photon becomes a signal. Here is the elegant chemistry. A photon strikes rhodopsin and twists its light-catching molecule (11-cis-retinal) into a new shape (all-trans-retinal). Activated rhodopsin switches on a G-protein called transducin, which switches on the enzyme PDE. PDE chews up a messenger molecule called cGMP. Here is the twist most people find surprising: in the dark, cGMP holds ion channels open, so the cell is relatively depolarized (about −40 mV) — a steady "dark current." When light makes cGMP fall, those channels close, the dark current stops, and the photoreceptor hyperpolarizes toward −70 mV. So a photoreceptor responds to light by going quieter, not louder — the opposite of most nerve cells.
8. Down the wire to the brain. That voltage change alters how much of the transmitter glutamate the photoreceptor releases onto bipolar cells, which pass the message to ganglion cells. The ganglion cells' long fibers all gather and leave the eye together as the optic nerve. At that exit — the optic disc — there are no rods or cones at all, so it is genuinely blind. You never notice this blind spot because your brain quietly fills the gap using the surrounding image and your other eye.
9. Accommodation — the lens changes shape. To keep both near and far objects in focus, a ring of ciliary muscle squeezes or relaxes the lens. For distant objects the lens is pulled thin; for near objects it bulges fatter to bend light more strongly. When the eyeball is a little too long or too short, or the lens stiffens with age (presbyopia), the focus point falls just in front of or behind the retina and the image blurs. That is where glasses and contact lenses come in: they add or subtract exactly enough bending to move the focus back onto the retina.