About Pupil
The human eye is made up of several external structures like the sclera, cornea, iris, pupil, lens, retina, optic nerves, cones, rods. The pupil comes under the external part of the human eye as it opens in the centre of the iris as the iris is the structure that gives colour to the eyes. The main function of the pupil is to allow light to enter the eye so it can be focused on the retina to begin the process of sight. They are perfectly round in shape, equal in size and of black colour.
Colour of Pupil
The colour of the pupil is mainly black because the light that passes through the pupil is absorbed by the retina and is not reflected back. In case the colour of the pupil is pale then the main reason behind this is the lens of the eye has become opaque due to the formation of a cataract. To obtain the colour of pupils bak, we have to remove the pale colour cataract by operating it. With the help of surgery pale lens is replaced by a clear intraocular lens.
There is one more situation when the colour of pupils get changed, is when someone takes your photo using the camera's flash. When taking a photo, the colour of the pupil may vary depending upon the direction of the gaze. The main reason behind this is intense light coming from the flash being reflected by the red colour of the retina.
Size of Pupil
The size of the pupil keeps on changing depending on the condition i.e. they tend to become bigger in low-intensity light which enables more light to enter into the eyes which make it easier to see anything and in high-intensity light, the pupil size becomes smaller or contracts. When a pupil is fully dilated its size is approx 4 to 8 millimetres and when it is constricted its size varies from 2 to 4 mm range. As per a recent study done by an American ophthalmologist, the normal size of the pupil is approx 2 to 8mm.
Functioning of Pupil
Iris and pupil together control the amount of light entering into the eyes. The pupil is the aperture of the eye and the iris is the diaphragm that controls or maintains the size of the aperture. Muscle within the iris is responsible for controlling the size of the pupil. One muscle constricts the pupil opening and another iris muscle dilates the pupil. This whole process of controlling the size of the pupil occurs within the iris. High-intensity light can cause glare and discomfort and sometimes it may damage the lens and retina.
Injury or Deformities which Cause Damage to Pupil
A. Concussion:
It is a kind of brain injury that occurs from the brain smacking against the hard skull during a fall, a hit to the head, which further shows an impact on the whole body. One of the common symptoms of this is an increase in the size of the pupil than normal.
B. Anisocoria:
In this condition, one pupil is wider than another, which can be because of both natural problems or may be accidental. This problem is found in almost 20% of people.
C. Coloboma:
In this condition, part of the eye doesn’t form the right way before you’re born.
D. Pituitary Gland Tumour:
As pituitary gland controls so many other glands that participate in making different hormones. So tumours in this gland can increase the size of pupils.
How Pupil of the Eye Regulate the Amount of Light?
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Let us see how the size of the pupil is changed to regulate the amount of light.There are muscles and connective tissue between the iris and the pupil. When bright light falls on the pupil, the circular muscles in the iris contract and the radial muscles relax which reduces the size of the pupil. Therefore, the amount of light passing through the pupil for a bright light is reduced and an image is formed on the retina comfortably. This is what happens when we go outside on a brighter sunny day, we will feel uncomfortable at first due to bright light reaching our eyes. Then the size of the pupil decreases and the amount of light passing through the pupil is also reduced. Then we can easily see clearly without any discomfort due to bright light. So, in bright conditions, we have a dilated pupil eye.
On the other hand, when it is dark and not clearly visible, the circular muscles relax and the radial muscles contact. This causes the pupil to dilate and the amount of light passing through the purple increases.This helps to maximise the vision of our eyes. A good example to understand this is when we go inside suddenly to a dark room. We will not be able to see clearly because of the low amount of light reaching our eyes. But as the time progresses, the pupil gets dilated and the amount of light passing through the pupil increases. Thereby increasing our vision at dim conditions.
Our emotions can also affect the size of the pupil.The pupil tends to dilate if we are delighted or surprised and the pupil size becomes small when we are angry. If we are in a fight or flight situation, the pupil dilates. In a situation of fight or flight, it is crucial that the vision of our eyes has to be very clear. To ensure that we are able to see properly, the pupil is dilated along with other sympathetic actions. The pupil of a dead person will always be dilated. To check whether a person is dead or alive, we can check the pupil of the person and if it is not constricting, then we can confirm that the person is dead.
Conclusion
The smaller circular opening of the iris is called pupil of the eye. The function of the pupil of the eye is to control the amount of light passing through the pupil. It is the responsibility of the iris to control the size of the pupil using circular and radial muscles. The contraction and relaxation of these muscles control the size of the pupil. The pupil constricts in bright conditions and the pupil of the eye dilates in dim conditions. The dilation and constriction of the pupil is also triggered by our emotions. The pupil dilates when we are surprised or pleased and the pupil constricts when the person is angry or doubtful. A dead person will have a dilated pupil eye.
FAQs on Pupil - Eye
1. Explain the Term Iritis?
Ans. Iritis is a condition when there is inflammation in the iris of the eyes which can be caused by infection, trauma, autoimmune diseases, etc. As we know iris controls the pupil, so this condition also leads to abnormalities in the pupils.
2. Which Types of Iris Muscle Controls the Size of Pupils?
Ans. As the colour of the pupil is mostly black, all the light that goes inside gets absorbed by the retina. It’s working totally like an aperture camera. There are two types of iris muscles that control the size of pupils, they are:
A. Circular sphincter pupillae – constricts pupil on contraction
B. Radial dilator pupillae – dilates pupil on contraction
3. What is the role of iris in the dilation and constriction of the pupil?
Ans: The amount of light passing through the pupil and reaching the retina of the eyes depends on the size of the pupil. The size of the pupil is controlled by the contraction and relaxation of circular and radial muscles in the iris. In bright conditions, the circular muscles contract and radial muscles contract and the pupil constricts. Hence, the size of the pupil is reduced and the amount of light passing through the pupil also decreases. In dim conditions, the circular muscles relax and radial muscles contract so that the pupil dilates. Therefore, the amount of light passing through the pupil also increases making the vision in dim conditions possible.
4. Why do people in photographs sometimes have red eye pupils?
Ans: When light falls on an object, we see the object in that colour that is reflected by the object. Thus, a red ball reflects red colour so we see the colour of the ball red. If most of the light is absorbed, then the colour of the object is black. The colour of the pupil eye is black because when the light passes through the pupil and reaches the retina, most of the light reaching the retina is absorbed.
While using a camera, we turn on the flash in dim conditions. In dim conditions , the pupil is dilated so that the amount of light passing through the pupil increases. But when the camera captures the picture by flashing the bright light just for a moment, there is not enough time for the pupil to constrict. This allows a large amount of flash light to pass through the pupil and reach the retina. The retina contains many blood vessels and when this bright flash light falls on the blood vessel on the retina, the red colour of the blood is reflected and the pupil of the person in the photographs looks red in colour.
