Definition
The iris is the colored, ring-shaped muscle at the front of the eye that surrounds the pupil and controls how much light enters by changing the pupil's diameter. In bright light it contracts to make the pupil smaller; in dim light it relaxes to make the pupil larger.
Plain English
It's the colored part of your eye. It works like a little muscle that opens and closes the black hole in the middle (the pupil) so the right amount of light gets in.
Context Anchor
Seen in night vision and eye anatomy discussions, especially when learning how the eye adjusts between bright light and darkness.
Derivation
From the Greek 'iris,' meaning rainbow, named for the variety of colors the iris can have between people. Knowing this links the word to its visible, colorful appearance in the eye.
Why Pilots Care
Understanding iris action explains why bright lights destroy night vision and why red cockpit lighting helps preserve it during night flight.
Analogy
Think of the iris as the aperture ring on a camera lens — it opens up in low light to let more in, and closes down in bright light to avoid overexposure.
Grounding Statement
In dim light, the iris opens the pupil wider; in bright light, it makes the pupil smaller.
Intuition Check
Do not think of the iris as only the color of the eye. Here, it is an active part of the eye that adjusts how much light gets in.
Example Sentence 1
Before a night flight, the pilot avoided bright lights so the iris would stay relaxed and the pupils wide open for better night vision.
Example Sentence 2
Bright headlights on the ramp caused the iris to contract, temporarily reducing the ability to see the taxiway.