Light energy is converted into nerve impulses in the a. iris b. retina c. optic nerve d. lens

It is said that your eyes are your windows to the world. However the actual act of “seeing” is much more complex than that common phrase would make it seem.

When you see an object what you are actually seeing are light rays reflecting off that object and into your eye. Your eye's complex components intercept, focus and process the light into nerve impulses, which are sent to the brain. This is how we "see."

Iris and Pupil

The iris is the part of your eye that gives it color (blue, green, brown, etc.) It functions like a shutter in a camera, allowing more or less light into the eye.

The pupil is the round opening in the middle of the iris.

Cornea and Lens

The cornea is the crystal clear dome that covers the front of your eye. When light passes through the pupil, the cornea bends (or refracts) the light rays.

The crystalline lens finishes the focusing of light. It helps to "fine tune" your vision.

Unlike the cornea, the lens can be made to change its shape (and therefore its refractive power) rapidly and voluntarily. By changing its shape, the lens allows your eyes to change focus and see objects that are near.

Retina

The retina is a thin layer of nerve tissue at the back of your eye that senses the light. Specialized cells called rods and cones convert light energy into nerve signals that travel through the optic nerve to the brain. Your brain then produces an image and the act of “seeing” is complete.

Vision Problems

The shape (curve) of the cornea, the power of the crystalline lens, and the length of your eye together determine how rays of light are focused on the retina.

When these 3 factors are sized and working appropriately, you can see clearly without the need for vision correction (glasses or contacts).

However, if some of these factors don’t match up, the result is vision impairment.

Nearsightedness (trouble seeing things that are far away) occurs when the cornea is too steeply curved or the eye is too long.

Farsightedness (trouble seeing things up close) occurs if the cornea is too flat or the eye is too short.

As you age, your crystalline lenses become stiff and lose the ability to focus on near objects (called presbyopia). This happens to everyone, usually around the age of 50, and results in the need for reading glasses, bifocals, contacts or a combination of these.

• Choroid
  Layer containing blood vessels that lines the back of the eye and is located between the retina (the inner light-sensitive layer) and the sclera (the outer white eye wall).  
• Ciliary Body
  Structure containing muscle and is located behind the iris, which focuses the lens.
• Cornea
  The clear front window of the eye which transmits and focuses (i.e., sharpness or clarity) light into the eye. Corrective laser surgery reshapes the cornea, changing the focus.
• Fovea
  The center of the macula which provides the sharp vision.
• Iris
  The colored part of the eye which helps regulate the amount of light entering the eye. When there is bright light, the iris closes the pupil to let in less light. And when there is low light, the iris opens up the pupil to let in more light.
• Lens
  Focuses light rays onto the retina. The lens is transparent, and can be replaced if necessary. Our lens deteriorates as we age, resulting in the need for reading glasses. Intraocular lenses are used to replace lenses clouded by cataracts.
• Macula
  The area in the retina that contains special light-sensitive cells. In the macula these light-sensitive cells allow us to see fine details clearly in the center of our visual field. The deterioration of the macula is a common condition as we get older (age related macular degeneration or ARMD).
• Optic Nerve
  A bundle of more than a million nerve fibers carrying visual messages from the retina to the brain. (In order to see, we must have light and our eyes must be connected to the brain.) Your brain actually controls what you see, since it combines images. The retina sees images upside down but the brain turns images right side up. This reversal of the images that we see is much like a mirror in a camera. Glaucoma is one of the most common eye conditions related to optic nerve damage.
• Pupil
  The dark center opening in the middle of the iris. The pupil changes size to adjust for the amount of light available (smaller for bright light and larger for low light). This opening and closing of light into the eye is much like the aperture in most 35 mm cameras which lets in more or less light depending upon the conditions.
• Retina
  The nerve layer lining the back of the eye. The retina senses light and creates electrical impulses that are sent through the optic nerve to the brain.
• Sclera
  The white outer coat of the eye, surrounding the iris.
• Vitreous Humor
  The, clear, gelatinous substance filling the central cavity of the eye.

How the Eye Works

The five senses include sight, sound, taste, hearing and touch. Sight, like the other senses is closely related to other parts of our anatomy. The eye is connected to the brain and dependent upon the brain to interpret what we see.

How we see depends upon the transfer of light. Light passes through the front of the eye (cornea) to the lens. The cornea and the lens help to focus the light rays onto the back of the eye (retina). The cells in the retina absorb and convert the light to electrochemical impulses which are transferred along the optic nerve and then to the brain.

The eye works much the same as a camera. The shutter of a camera can close or open depending upon the amount of light needed to expose the film in the back of the camera. The eye, like the camera shutter, operates in the same way. The iris and the pupil control how much light to let into the back of the eye. When it is very dark, our pupils are very large, letting in more light. The lens of a camera is able to focus on objects far away and up close with the help of mirrors and other mechanical devices. The lens of the eye helps us to focus but sometimes needs some additional help in order to focus clearly. Glasses, contact lenses, and artificial lenses all help us to see more clearly.