Key ideas

• Light is bent as it enters through the cornea, and the adjustable lens fine-tunes the focus so the image lands on the retina.
• Through accommodation, eye muscles change the lens's shape so you can focus on near or far objects, which is why the other distance blurs.
• Nearsightedness and farsightedness happen when the eye focuses the image in front of or behind the retina instead of on it.
• Rods work best in low light and detect shapes and movement, especially in peripheral vision, but do not provide color.
• Cones provide color and fine detail in brighter light and are concentrated in the fovea, so looking directly at something gives the sharpest vision.
• Where the optic nerve exits the eye there are no photoreceptors, creating a blind spot that the brain fills in using surrounding visual information.

Key terms

retina

The light-sensitive surface at the back of the eye, lined with photoreceptors.

photoreceptors

Special cells in the retina that respond to light and transduce it into neural signals.

cornea

The front of the eye where light enters and first bends; unlike the lens, it is not adjustable.

lens

The adjustable structure that changes shape to fine-tune focus of the image on the retina.

accommodation

The change in the lens's shape, controlled by eye muscles, to focus on near or far objects.

nearsightedness

Light from distant objects is focused in front of the retina, so distant objects blur but near ones are clear.

farsightedness

Light from nearby objects is focused behind the retina, so near objects blur but distant ones are clear.

rods

Photoreceptors, mainly in the retina's periphery, that are sensitive in low light and detect shapes and movement but not color.

cones

Photoreceptors that provide color and fine detail and work best in bright light (C = Color, Clarity, Center).

fovea

A small area near the center of the retina with a high concentration of cones, giving sharp, detailed vision.

blind spot

The spot where the optic nerve exits the eye; it has no photoreceptors, so that part of the visual world is not detected.

Watch out