Blue eye without visual aids

Anatomy of the Eye

Emmetropia: Perfectly focused vision

With emmetropia or normal vision, light emitted or reflected from objects travels through the parts of your eye, forming a clear and sharp image.

Parts of the Eye

The eye is a complex structure with each part working together to create an image through the conversion of incoming visual information into sight by the brain. The eye can be divided into three parts: Front, inner and back.

  • Front of the Eye

    1. Sclera: The sclera is the white, outer area that creates and supports the structure of the eye.

    2. Cornea: The cornea is the clear, front part of the eye that bends light as it enters the eye.

    3. Pupil: The pupil is the opening in the center of the iris, where light passes through.

    4. Iris: The iris controls the size of the pupil, which in turn regulates the amount of light entering the eye.

    5. Conjunctiva: Conjunctiva is the clear covering of the sclera that helps keep the outer eye healthy.

    Outside of the eye are the eyelashes and the eyelid. The average person blinks about 12 times per minute to keep the eye lubricated. Eyelids and eyelashes also protect the eye from dust and other particles that could scratch the eye. Visible parts at the front of the eye are the cornea, the pupil, the iris, the sclera and the conjunctiva.

    The cornea is comprised of the first transparent layers of the eye and strongly refracts incoming light. The light then passes into the inside of the eye through the round opening called the pupil. The iris, the colored part of the eye, opens and closes to change the size of the pupil, which in turn regulates the brightness of the incoming light.

  • Inner Eye

    6. Natural Lens: The natural lens is the clear structure in the eye behind the iris that helps to focus light onto the retina. The lens is able to change its shape so the eye can focus on objects at various distances. This focusing ability of the lens is known as accommodation.

    7. Vitreous Body: The vitreous body is the clear, jelly-like substance that fills the inner eye (vitreous cavity). It helps maintain the shape of the eye.

    After light passes through the pupil it reaches the natural lens. This marks what can be considered the inner portion of the eye, which ends at the retina. Once light hits the natural lens, the lens can change shape to accommodate the distance between the eye and the observed object, and focuses the light into a single point on the back of the eye. The space between the natural lens and the back of the eye is called the vitreous cavity.

  • Back of the Eye

    8. Retina: The retina is a thin layer of light-sensing cells lining the back of the eye.

    9. Macula: The macula is the area on the retina where we have our central vision. It is a small, orange area with a high concentration of photoreceptors, which are sensitive to light. It allows you to have detailed central vision, which is imperative for activities like reading, and appreciating color.

    10. Optic Nerve: The optic nerve is a bundle of nerve fibers at the back of the eye that transfers visual information from the retina to the brain.

    Lining the back of the eye is the retina. The retina consists of millions of receptors that receive the incoming light rays. These receptors convert the light rays into electrical impulses that then travel through the optic nerve to the brain, where they become images.

    Attached to the eye are six muscles known as the extraocular muscles. They are among the most active muscles in your body and allow both eyes to move in unison without turning your head. Coordinated by the brain, this group of muscles can contract in less than 1/100th of a second to help your eye quickly track moving objects.

How Vision Works

Starting with light and ending in sight
Vision begins with the cornea, at the front of the eye, since it refracts incoming light into the pupil, the dark opening in the eye. The iris, the colored ring surrounding the pupil, opens and closes to control the amount of light entering through the pupil. Once past the pupil, the light passes through the lens, which further refracts the light onto the back of the eye.

Transferring visual information to the brain
A thin layer of tissue called the retina covers the back of the eye. The retina is made of millions of receptors and nerves that detect the image formed by the optics of the eye much like the pixels of a digital camera. These nerves send information to the optic nerve, which transfers information to the brain. If both eyes function properly, information from both eyes is merged together into a single, three-dimensional image by the brain, allowing us to see the world with a sense of depth.

Perfectly focused vision
The condition resulting in normal, perfectly focused vision is called emmetropia, derived from a Greek word meaning equally measured. The proportions of the eye have to be exact so that light is brought into sharp focus at the back of the eye. Where the focal point lands, in reference to the lens, determines the eye’s refractive power. Your refractive power is measured in diopter, which is 0 dpt for emmetropia.