The eye is a complex organ that manipulates light much like a camera does; it has lenses to change the focus and a sensor to recognize intensity and color.
How Does the Eye Focus? Starting with the Cornea
The first layer of the eye that light hits is the cornea, the surface of the eye. The cornea is a dome-shaped lens that starts the process of focusing light, contributing approximately two-thirds of the eye's focusing power. But the cornea is like the lens of eyeglasses - it always refracts light the same amount, unlike the lens of a camera which can focus at different depths.
The shape of the cornea is maintained by the aqueous humour, a gel that lies between the cornea and the lens.
The Pupil and Iris Regulate Amount of Light
The iris is the colorful part of the eye. The pupil, the black spot in the middle of the iris, is actually just a hole in the iris, which can contract or relax to adjust the size of the pupil. In low light, the pupil expands to allow more light into the eye. In bright light, it contracts to protect the eye and increase contrast.
The Lens Focuses Light
Behind the pupil lies the crystalline lens, which is responsible for focusing light. The lens can change its focal length, like a camera. This is what allows you to focus close up on a book, or far away at the horizon, but not both at the same time. To switch from one to the other, the lens actually bends and changes shape because of contractions in the cilliary muscles.
A healthy lens is critical to good vision. As people age, the lens can become cloudy, causing a cataract, or stiff, causing presbyopia. When the lens becomes stiff, the cilliary muscles can no longer change the shape of the lens to focus on up close objects.
The Retina Detects Light
From the crystalline lens, light travels through another gel known as the vitreous humor, which maintains the shape of the eye, to the retina in the back of the eye. The retina contains light-receptor cells known as rods and cones. Rods are very sensitive and simply detect light, giving us our nearly-colorless night vision, while cones detect different colors. Cones are concentrated in the fovea, a pit in the center of the retina, providing very sharp central vision.
Oddities of the Retina: Flipped and Holey
The lens projects an image onto the retina, but it is rotated 180 degrees (upside down and backwards). If you flip upside down to watch a movie, what you see is actually what is being projected onto the retinas of your bemused friends. This is because the shape of the lens causes light to converge through a single point inside the lens, emerging out the back like light leaving a projector.
There is also a gap in your vision known as the blind spot, where blood vessels and nerves pass through the retina. So why don't you see a flipped world with a hole in it? The brain corrects for both of these, providing you with a properly oriented image and filling in the blind spot with the surrounding color.
To recap: light is partially focused when it passes through the cornea, then travels through the aqueous humour to the crystalline lens, which lets the eye focus on different depths. The light converges in the lens and travels out the other side flipped, traveling through the vitreous humour to the retina on the inner back surface of the eye, where rods and cones detect light. Then your brain presents a coherent, correctly-oriented image.
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