Humans eyes work much like cameras. Here is a simple step-by-step explanation of how the human eye works and a look at the structure and function of the parts of the eye.
Parts of the Eye and Their Functions
To understand how the human eye works, you need to know the names and functions of its structures.
- Cornea: The cornea is the transparent outer surface of the eye. Because the eyeball is round, the cornea acts as a lens that bends or refracts light. Corneal cells regenerate quickly, because the cornea is exposed to the environment. But, the layer is thin enough to allow oxygen into the deeper structures.
- Aqueous Humor: The aqueous humor is the fluid layer below the cornea. It has a composition similar to human plasma. The aqueous human shapes the cornea and nourishes the cells of the eye.
- Iris and Pupil: Light passes through the cornea and aqueous humor through a hole called the pupil. The iris is a contractile ring that determines eye color and controls the size of the pupil. The iris dilates (opens) the pupil in low light so more light enters the eye and constricts in bright light.
- Lens: While the cornea initially focuses light, the lens makes it so you can change focus between near and distant objects. Ciliary muscles around the lens contract to thicken the lens to focus on near objects. The muscles relax to flatten the lens to focus on distant objects.
- Vitreous Humor: The vitreous humor is a transparent gel that fills the eye. It supports the shape of the eye and provides enough distance so that the lens can focus.
- Retina: The retina is the coating on the inside of the back of the eye. It contains two types of cells. Rods detect light and help form images in dim light. Cones detect colors. There are three types of cones. They are called red, green, and blue cones, but they actually detect a range of wavelengths of light and not just the colors for which they are named.
- Fovea: The fovea is the circle of cells on the retina responsible for clear focus. This region is rich with cones, so it allows sharp color vision. Rods outside the fovea are largely responsible for peripheral vision.
- Optic Nerve: Light striking a rod or cone produces an electrochemical signal. The cells transmit this signal through the optic nerve to the brain.
- Brain: The visual cortex of the brain receives nerve impulses from both eyes and compares them to construct a three-dimensional image. Because the eye is like a camera, the true image formed on the retina is inverted (upside down). The brain automatically rights the image.
How the Human Eye Works
Now that you know the names of the parts of the eye, it’s easy to follow the steps leading to vision.
- Cornea: Light enters the eye through the cornea. Because of the shape of the cornea, it exits pre-focused.
- Aqueous Humor/Pupil: From the cornea, light passes through the aqueous humor and through the pupil.
- Lens: From here, light strikes the lens. The lens further focuses light, depending on whether you’re looking at a near or distant object. Light exits the lens and passes through the vitreous humor.
- Vitreous Humor: Ideally, the vitreous humor is clear and allows light to travel unimpeded to the retina.
- Retina: Light reaches the retina, activating rods and cones to generate electrical impulses that code for an inverted image.
- Optic Nerve: Signals from the rods and cones travel through the optic nerve to the brain.
- Brain: The brain compares left/right vision to add depth and make the image three-dimensional. It also flips the image so it appears right-side up.
Common Eye Problems
The most common eye problems are myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. These conditions affect vision, but the eyes may be perfectly healthy.
- Myopia: Nearsightedness occurs when the focal point of the eye is in front of the retina. In other words, the eye is narrow rather than spherical.
- Hyperopia: Farsightedness occurs when the focal point of the eye is past the retina. In other words, the eye is slightly flattened rather than spherical.
- Presbyopia: Presbyopia is age-related farsightedness. It’s caused by stiffening of the eye’s lens over time. Presbyopia often improves myopia.
- Astigmatism: Astigmatism occurs when the eye curvature isn’t perfectly spherical. This makes light focus unevenly from one part of the eye to another.
Other common eye problems include glaucoma, cataracts, and macular degeneration. These conditions can lead to blindness.
- Cataracts: Cataracts are clouding and hardening of the lens.
- Macular Degeneration: Macular degeneration is progressive degeneration of the retina.
- Glaucoma: Glaucoma is increased fluid pressure within the eye. This can damage the optic nerve.
Interesting Eye Facts
Here are some fun and interesting eye facts you may not know:
- Babies are born with full-sized eyes. Eye size remains the same from birth until death.
- Blind people with eyes may still be able to sense light and dark. This is because there are cells in the eyes that detect light, but aren’t involved in image formation.
- Each eye has a blind spot where the eye attaches to the optic nerve. If you close one eye, you can find the blind spot. Normally, the second eye compensates and fills in the hole in your vision.
- The reason total eye transplants aren’t possible is because it’s presently too difficult to make the million-plus connections in the optic nerve.
- Humans don’t ordinarily see ultraviolet light, but the retina can detect it. The lens absorbs UV light before it reaches the retina, presumably to protect it from the high energy light capable of damaging rods and cones. However, people with artificial lenses report seeing ultraviolet.
- Blue eyes don’t contain any blue pigment. Instead, they lack pigment found in other eye colors. Rayleigh scattering of light causes the blue color in the same way as it makes the sky appear blue.
- Eye color can change over time. Usually, color change occurs from hormonal changes or chemical reactions from medications.
References
- Bito, L. Z.; Matheny, A.; Cruickshanks, K. J.; Nondahl, D. M.; Carino, O. B. (1997). “Eye Color Changes Past Early Childhood”. Archives of Ophthalmology. 115 (5): 659–63.
- Goldsmith, T. H. (1990). “Optimization, Constraint, and History in the Evolution of Eyes”. The Quarterly Review of Biology. 65(3): 281–322.