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The Pupil: What It Is & Why We Have it

Jamie Mendiola

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Jamie Mendiola

Updated: 12 June 2024 •  
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The pupils of your eyes – the little black dots at the center of your eyes – play a big part in making sure you can see every day.
While they might look tiny and get easily ignored, the pupils represent your eye health in the long run. Keep on scrolling down and see why they should be a priority in your next eye check-up.

What is the pupil?

pupil in brown eye

The pupils work like little openings in your eyes.

Grab a mirror and look closely into your eye. Can you see the pupil?
The pupil is a tiny opening or hole in the center of your iris, the colored area of your eye. This little circle controls how much light can enter your eye so that you can see your surroundings correctly.
Because of its function, our pupils often dilate or contract when it reacts to light. They’re pretty similar to a shutter in a camera. The pupils constantly adjust so your eyes can focus and perceive an object.
Our eyes’ pupils usually look like mini black holes, equal in size for both eyeballs. However, if the pupils are hazy or pale, your eyes may have developed a cataract.
Another scenario is when you take a selfie with a flash, and your pupils might appear red in the photo. This effect is due to intense light reflecting on the red of your retinas. That’s the part behind the pupils and lens of your eyes.

How the pupil works

human eye pupil reacting to light

The pupils receive and react to light.

Imagine your eyes as a camera. It has multiple parts that move like a well-oiled machine so you can perceive the environment around you in vivid HD. And one of the most important things you need for clear vision is light.
If your eyes didn’t receive light, your vision would be gone. To solve that, two segments in your eyes receive light: the iris and the pupils.
The iris works like a control plate that houses your pupils. It has two muscles: the dilator muscle, which increases pupil size, and the sphincter muscle, which does the opposite.
As the iris muscles change shape, the pupils dilate or contract, allowing light to enter your eye. The lens behind the pupils will focus on this until the light hits the back of your eye. That, in turn goes onto your retina.
The retina interprets the light as signals and sends them to your brain. Then, the brain will process these signals as images. This whole process is how you’re able to see, and it takes only a dozen milliseconds.
Also, remember how we mentioned that the pupils are tiny black holes? Yeah, they don’t just receive light. The pupils also provide a two-way street for aqueous humor, a thin, transparent liquid that nourishes the front of your eyeball and keeps it inflated.
In contrast, vitreous humor is another gel-like fluid inside your eye. It keeps your eye nourished and makes sure the light reaches your retina. However, microscopic collagen fibers might clump in that fluid as you age, creating shadows called eye floaters.

Pupil testing in an eye exam

eye doctor shining a light on male patient pupil

Testing the pupils often requires a light flash at your eyes.

When you get your eyes checked out, the eye doctor will inspect your pupils and how they work. Typically, pupil testing is quick and painless.
For instance, you might have to sit in a dimly lit room and look at an object. Then, the eye doctor will quickly flash a light at your eyes, testing how your pupils react. This practice is often called the Marcus Gunn pupil testing or the “swinging flashlight test.”
When necessary, they might also do other types of pupil testing. Examples would be using medicated eye drops to dilate the pupil for further examination or blood tests.
These pupil testing alternatives occur if the eye doctor detects some irregularities in your test result and wants a more thorough diagnosis.

4 common diseases that affect the pupils

pupil with abnormal dilation

Certain eye conditions can dilate or constrict the pupils for a long time.


1. Marcus gunn pupil

Remember the “swinging flashlight test” mentioned earlier? If the pupils don’t react as expected to the light, you might get diagnosed with Marcus Gunn pupil. The condition is also known as an afferent pupillary defect (RAPD).
With this condition, your eyes will look dilated before a light source. A typical cause of this is damage to your optic nerve or a disease affecting the retina.

2. Anisocoria

If your pupils are unequal in size, it might be anisocoria. That’s a relatively common condition, affecting around 20% of adults today. It’s either benign or a symptom of a life-threatening head injury.

3. Adie syndrome

Also called Adie’s tonic pupil, this syndrome is when the pupil has zero or delayed reaction to light. It only affects one eye and can be caused by trauma, lack of blood flow, or an infection.

4. Miosis

While some conditions dilate the pupil excessively, miosis does the opposite. This condition constricts the pupils, making them appear smaller than usual. It can be due to drugs, Horner’s syndrome, injury, or other severe inflammatory conditions.

4 ways to care for the pupils

1. Quit or avoid smoking

We all know that smoking is terrible for your lungs, but did you know? It can also damage your eyes.
Smoking increases the risk of macular degeneration, glaucoma, diabetic retinopathy, and dry eye syndrome. It can make your pupils smaller and decreases retinal activity, which can lead to eventual vision loss.
Smokers are also two to three times more likely to get cataracts than those who never touched a cigarette. If you want to keep your eyes healthy, lay off the cigars.

2. Consume a diet rich in eye-friendly nutrients

A healthy diet can create a big difference in your vision. It’s not just about eating more carrots to make your eyesight sharper but also about eating balanced meals.
When possible, include these dietary options in your meal plan;

  • Green leafy vegetables
  • Citrus fruits
  • Seafood rich in omega-3 oils
  • Walnuts
  • Flax seeds
  • Sweet potatoes
  • At least 8 cups of water daily


3. Use protective eyewear

Whether you’re working in your home office or doing manual labor outside, never forget to wear protective eyewear. It can save your eyes, and your life, in certain situations.
For instance, if you’re in an area with so much dust and debris, a good pair of prescription safety glasses can ensure your vision’s clear while protecting your eyes from the elements.

4. Get regular eye exams

An eye exam shouldn’t be a rare, once-in-a-blue-moon type of event. It should be part of your routine. You must get an optical check-up every two years to keep your eyes healthy and safe.

Pupils are tiny markers of your eye health

Your pupils might be a tiny part of your eyeball, but they play an integral role in your vision. Without them, you won’t be able to see your surroundings at all.
Take care of your pupils starting today. A pair of glasses and regular eye checkups can save you a lot of hassle in the long run. If you enjoy this article, learn more about caring for your eyes on Mouqy’s blog.

Jamie Mendiola
Jamie is a writer and non-profit volunteer advocating for mental health. When not typing up a storm at work, you'll find her binge-watching Netflix or meditating like Yoda.