It's a well-known fact that cats can adjust the size of their pupils.

You may notice that during the day, the part of the eye called the pupil becomes a narrow slit. As the sun weakens, the cat's pupil gradually enlarges until it becomes a large, round shape at night.

This fascinating phenomenon is a way for cats to regulate the amount of light entering their eyes. When the sun is too bright, the pupil contracts to protect their eyes from excessive light.

On the other hand, when there's not enough light, the pupil expands to enhance their vision and observe their surroundings.

There have been videos circulating showing cats' pupils rapidly dilating upon seeing the owner. Curious to explore this phenomenon, owners have attempted to replicate the experiment.

The cats' eyes didn't become round when they saw their owners. Instead, their pupils quickly dilated upon seeing a finger or the teaser stick, followed by a swift pounce or bite.

This observation suggests that a cat's pupils become round when they encounter something of interest or excitement, similar to their response to cat food or favorite toys. The size of a cat's pupils can adjust automatically, not solely in response to light.

For instance, under the strong sun at noon, a cat's eyes naturally squint into narrow slits. When the cat is in a good mood, their eyes may appear half-closed, while a bad mood might be conveyed with a "Don't Disturb" look.

At night, even in a well-lit room, a cat's eyes are often fully open, without the squinting seen during the day. It's worth noting that not only cats but humans also experience pupil dilation when they see someone or something they adore.

In ancient times, Persian merchants even used pupil dilation as an indicator to price gemstones.

A cat's pupils are not only influenced by light but also by their own emotions. Excitement, agitation, nervousness, or fear can trigger an adrenaline rush, causing the pupils to dilate.

Once the cat returns to a calm state, the pupils will revert to their original size.

When playing with toys, cats become stimulated and exhibit a hunting-like excitement, resulting in dilated pupils that allow them to focus on the details.

Similarly, when taken to an unfamiliar environment, a cat may feel nervous or scared, leading to wide-open pupils.

As for why a cat's pupils may not dilate upon seeing their owners, it could be due to familiarity. Cats may have grown accustomed to seeing their owners every day, similar to how we may not feel amazed by something we see repeatedly. If you try to test it by surprising the cat after a two-day absence, you might witness their pupils dilating at the moment of reunion.

While large and round pupils are normal for cats, it's important to monitor their eye condition.

If a cat's pupils are consistently dilated or always squinted into narrow slits regardless of sunlight, it may be necessary to consult a veterinarian.

Chronic kidney disease, hypothyroidism, or eye conditions like cataracts could be potential underlying causes, and timely medical attention is crucial in such cases.