The iris controls the amount of light entering the eyes. The iris automatically adjusts the size of the pupil according to the intensity of light received by the eye. If the amount of light received by the eye is large (as during the day time), then the iris contracts the pupil (makes the pupil small) and reduces the amount of light entering the eye (see Figure).
On the other hand, if the amount of light received by the eye is small (as in a dark room or during night), the iris expands the pupil (makes the pupil large) so that more light may enter the eyes (see Figure).
Thus, the iris regulates (or controls) the amount of light entering the eye by changing the size of the pupil. The iris makes the pupil ‘expand’ or ‘contract’ according to the intensity of light around the eye. If the intensity of the outside light is low, then the pupil expands to allow more light to enter the eye. On the other hand, if outside intensity of light is high, then the pupil contracts so that less light enters the eye.
It should be noted that the adjustment of the size of the pupil takes some time. For example, when we go from a bright light to a darkened cinema hall, at first we cannot see our surroundings clearly. After a short time our vision improves, and we can see the persons sitting around us. This is due to the fact that in bright sunlight the pupil of our eye is small. So, when we enter the darkened cinema hall, very little light enters our eye and we cannot see properly. After a short time, the pupil of our eye expands and becomes large. More light then enters our eye and we can see clearly. On the other hand, if we go from a dark room into bright sunlight or switch on a bright lamp, then we feel the glare in our eyes. This is due to the fact that in a dark room, the pupil of our eye is large. So, when we go out from a dark room into bright sunlight or switch on a bright lamp, a large amount of light enters our eyes and we feel the glare. Gradually, the pupil of our eye contracts. Less light then enters our eye and we can see clearly. In this way, the iris also protects our eyes from the glare of bright lights.
Frequently Asked Questions (FAQs)
1. What is the iris, and where is it located in the eye?
- The iris is the colored part of the eye that surrounds the pupil. It’s a ring-shaped structure located between the cornea and the lens.
2. What is the primary function of the iris?
- The iris controls the size of the pupil, which regulates the amount of light entering the eye. It adjusts the pupil’s diameter based on lighting conditions to optimize visual clarity.
3. How does the iris change the size of the pupil?
- The iris contains muscles that contract and expand to change the size of the pupil. In bright light, the iris constricts the pupil to reduce the amount of light entering the eye. In dim light, the iris dilates the pupil to allow more light in.
4. Why do our pupils dilate when we are in low-light conditions?
- Pupil dilation in low-light conditions is a protective mechanism to allow more light to reach the retina, improving our ability to see in the dark.
5. Can the iris change color over time?
- Yes, the color of the iris can change gradually over a person’s lifetime. This is often due to genetic factors and may result in subtle changes in eye color.
6. What is anisocoria, and when should I be concerned about it?
- Anisocoria is a condition in which the pupils are different sizes. Mild anisocoria can be normal, but significant and sudden differences in pupil size can indicate an underlying medical issue and should be evaluated by a healthcare professional.
7. Can emotions or arousal affect the size of the pupils?
- Yes, emotions and arousal can influence pupil size. Excitement or arousal can cause pupil dilation, while stress or fear can lead to pupil constriction. This is often a physiological response known as the “fight or flight” response.
8. How does medication or drugs impact the size of the pupils?
- Certain medications and drugs can affect pupil size. For example, some drugs can cause pupil dilation as a side effect, while others may lead to pupil constriction. Healthcare providers often use pupil reactions to assess a patient’s neurological status.
9. Can the iris be affected by medical conditions or diseases?
- Yes, medical conditions such as glaucoma and certain neurological disorders can affect the iris and its function. These conditions may lead to abnormal pupil size or shape.
10. Can the color of the iris impact a person’s susceptibility to certain eye conditions?
- There is some evidence to suggest that people with lighter-colored irises may be more susceptible to certain eye conditions like cataracts and age-related macular degeneration, although genetics and other factors play a significant role.