Eye Facts

FYI about your Eyes 

Do you know why you can’t see well underwater, Why your eyes sometimes look red in a photograph, Why your eyelid sometimes twitches uncontrollably? These interesting questions and more are answered here- “For your information about your Eyes. ” Did you ever wonder. . ..?

How can two brown-eyed parents give birth to a blue-eyed child?

The color of your eyes is determined by the genes you inherit from your parents. A gene for brown eyes is dominant. This means it over-rides genes for other eye colors. The gene for blue eyes is recessive. It shows up only when there are no genes for other eye colors. Someone with brown eyes may have a recessive, or “hidden,” gene for blue eyes. Therefore, two brown-eyed parents may each give a recessive gene for blue eyes to their child, who would then have blue eyes. However, you can’t necessarily tell a baby’s eye color at birth. Often, babies are born with blue eyes, which darken in the first few months of life. Gray, green and other eye colors result from a complex mixing of different eye color genes.

Why do I see spots after looking at a bright light?

Intense light, such as from a flashbulb or uncovered light bulb, can stimulate the retina (the light-sensitive tissue at the back of the eye) to its maximum response. When this occurs, it takes awhile for the retina to “recover.” For a short time you are not able to see well, or you may see spots or a dark afterimage.

Is it possible to tell what someone is feeling by looking at only his or her eyes?

The pupil (the black circle in the middle of the colored iris) may indicate what someone is feeling. Several factors determine the size of the pupil. Primarily, the pupils dilate (get bigger) or constrict (get smaller) to control the amount of light that enters the eyes. In addition, emotions can change the size of your pupils. When you experience pleasure, your pupils briefly dilate. Anger and fear can cause the pupils to constrict.

Pleasure vs. Anger illustration

© University of Illinois Board of Trustees Usage without written permission is prohibited.

Why does my nose run when I cry?

Tears leave the surface of your eye by evaporating or by entering the tear drainage system. A thin tube called the Canaliculus drains the tears from the inner corner of the eyelids down into the nose. Therefore, when your eyes fill with tears, such as when you cry or cut an onion, most of that fluid ends up in your nose.

Why do my eyes sometimes look red in a photograph?

If you look directly at the camera during a flash photo, the light from the flash entering your eyes may reflect off the retina, as if reflecting off a mirror. The back of the eye contains many small blood vessels, which give it a reddish appearance. Therefore, the reflected light looks red. To prevent this reflection from ruining their photographs, professional photographers use a flash that is not exactly in line with the camera. Amateur photographers can ask their subjects to look slightly to the side of the camera.

Can my contact lens get lost behind my eyeball?

A contact lens that slips off the corneal surface of the eye and moves under the eyelid cannot go far. The conjunctiva, a clear lining that covers the inner surface of the eyelids, forms a small sac or pouch between the eyelid and the eye to contain the eye’s tears. The “lost” contact lens can go only as far as the edge of this sac, not behind the eyeball. Usually you can gently manipulate the contact lens back into place. If the lens is difficult to retrieve, you may need to flush your eye with sterile saline. Your eye doctor can advise you on safe methods of retrieving a “lost” contact lens.

The conjuctiva can be compared to the pouch on a kangaroo

© University of Illinois Board of Trustees Usage without written permission is prohibited.

Why does my eyelid occasionally twitch uncontrollably?

Sometimes part of your eyelid may twitch for several minutes or hours. An involuntary twitching of a muscle is called Myokymia. Myokymia of the eyelid can occur with stress, lack of sleep or too much caffeine. Occasional twitching of an eyelid usually disappears by itself and does not indicate a serious problem unless it persists or extends to other parts of the face.

Why might looking toward a bright light cause a person to sneeze?

The “Photic sneeze,” a sneeze reflex related to bright light, is not completely understood. As bright light stimulates the retina, it most likely stimulates other related parts of the nervous system. Scientists believe that this stimulation causes nasal congestion, producing a sneeze in some people.

Will looking at the sun really hurt my eyes?

Occasionally glancing at the sun usually does not harm your eyes. However, staring for several minutes at the sun or a solar eclipse can damage visual cells in the part of the retina that allows us to see fine detail. This kind of injury, called solar retinopathy, can cause a blind spot in the center of the field of vision. Loss of vision in these cases can be temporary but often is permanent.

How big is my eyeball?

The human eyeball measures less than one inch in diameter. It is not completely round but is shaped more like a slightly flattened ball. The size of a normal eye differs only a little from person to person but can appear very different depending on the structure of each individual’s eyelids.

Why does vision blur underwater?

The surface of the eye bends light rays, focusing them on the retina and enabling us to see clearly. Light rays bend as they pass from air outside the eye to the fluid within the eye.When someone’s eyes are open underwater, the density of the water at the surface of the eye is similar to that of the fluid inside the eye. This decreases the ability of the eye to bend light rays to a focus, and vision is blurry. Wearing a diving mask keeps air in front of the eye, which provides an air-fluid difference and enables the diver to see normally.

 “Eye Facts” is an informational series and should not be used as a substitute for medical advice. For eye appointments, call (312) 996-6591. All Eye Facts illustrations and images are copyright protected and are the property of the UIC Board of Trustees. Unauthorized use of the images is prohibited. For usage of any Eye Facts content or illustrations please contact the Office of Medical Illustration at eyeweb@uic.edu or 312-996-5309 for licensing.
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