When the bright light of a camera flash shines directly through the pupil, it can reflect off the choroid , which supplies red blood to the retina the light-sensitive lining at the back of your eye , and bounce right back out through the pupil. If this happens, the person in the photograph will appear to have glowing red eyes. To avoid this, photographers move the flash away from the camera lens. With this arrangement, the light from the flash goes through the pupil at an angle, illuminating a part of the retina not captured by the camera lens.
Many cameras are equipped with red-eye reduction features, such as a pre-flash that causes pupil constriction before the actual flash that illuminates the photo.
This Science Snack is part of a collection that highlights Black artists, scientists, inventors, and thinkers whose work aids or expands our understanding of the phenomena explored in the Snack. Patricia Bath , pictured above, was an ophthalmologist and laser scientist, and was the first woman chair of ophthalmology at a US university.
She studied the causes of and cures for blindness, and invented a widely used method of using laser surgery to treat blindness caused by cataracts. Bath also co-founded the American Institute for the Prevention of Blindness.
This Science Snack can help you investigate the structures in the eye that help you see, so you can understand the eye like Dr. Bath did. Attribution: Exploratorium Teacher Institute. Connect with us! Get at-home activities and learning tools delivered straight to your inbox. The Exploratorium is a c 3 nonprofit organization. Pupil Your pupil changes size to control how much light enters your eye. Grade Bands:.
Scale, Proportion, and Quantity. Structure and Function. Some of these other factors include:. A concussion is a brain injury that results from the brain smacking against the hard skull during a fall, a hit to the head, or a fast impact involving the whole body.
One symptom is bigger-than-normal pupils. In some cases, one pupil may be bigger and the other smaller asymmetrical. Anisocoria is a condition in which one pupil is wider than other. While it can be a natural occurrence, affecting about 20 percent of people, it can also signal a nerve problem or infection. This is an intensely painful headache that usually affects one side of the face, directly behind the eye. As the name implies, it occurs in clusters sometimes as many as eight headaches a day , and can then disappear for weeks or months at a time.
Because this type of headache affects nerves in the face, the pupil on the affected side can become abnormally small called miosis during the headaches. This is an inflammation of the iris of the eye that can be caused by infection, trauma, and autoimmune diseases diseases in which your body attacks its own immune system.
According to research in the Emergency Medicine Journal , the pupil is typically smaller than normal. That injury can cause pupils to become smaller. Some causes include:. Certain drugs can dilate pupils while others constrict them. Some drugs that affect pupil size include:. Parts of the brain that help us feel and decode emotion as well as mentally focus can make pupils widen. How often you get your vision checked depends on your age and certain health factors.
But overall, most adults should have their vision checked every couple of years. Most people have pupils that are only a couple of millimeters wide and symmetrical meaning both eyes have the same size pupil.
You need healthy pupils to see properly. It is very similar to a camera aperture which allows more light in for more exposure. At night, our pupils dilate to allow more light in to maximize our vision.
In the bright sunlight, our pupil shrinks to a very small diameter to allow us to function normally. Otherwise, we would be very light sensitive. This protects the sensitive photoreceptors in our retina. Also, when we look at something at a very close distance such as reading a book, our eyes converge and our pupils shrink. When our pupils shrink, it is similar to looking through a pinhole. Looking through a small hole reduces peripheral blur and increases the depth of focus.
This improves overall visual acuity. Normal pupil size is between 2. The iris , the colored part of our eye, is made up of pigment and contains two sets of smooth muscles that control the size of the pupil: the sphincter muscle and the dilator muscle. The sphincter muscle is in the shape of a ring at the margin of the pupil. When it contracts, it constricts or decreases the size of the pupil. The dilator muscles are in a radial shape throughout the iris and when it contracts, it dilates or increases the size of the pupil.
Both systems, the parasympathetic and sympathetic systems control the pupil. Our parasympathetic system controls everyday activities such as rest, slowing the heart rate and things like digestion. It controls the pupil size during normal activities during the day and acts to change the pupil size depending on how much light is present. The sympathetic system is a protective system and gives us the typical "fight or flight" responses. In the pupil, if we are scared or feel fear, our pupils dilate very large.
This is thought to allow light in so our responses are quicker. Examination of the pupil is important because it can denote problems in the pathways controlling the pupil.
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