Plain English Meaning
The part of the eye that gets bigger or smaller to let in more or less light, the same way a camera lens opens wider in the dark and closes down in bright sun.
Full Definition
In the context of the eye, the diaphragm is the iris — the colored, adjustable structure that opens and closes around the pupil to control how much light reaches the retina. It functions like an aperture, expanding in low light and contracting in bright light.
Context Anchor
Seen in night vision discussions when comparing the eye to a camera and explaining how the eye adjusts to changing light.
Derivation
From the Greek 'diaphragma,' meaning 'partition' or 'barrier across.' In anatomy and optics, it refers to any structure that divides a space and regulates what passes through — in the eye, what passes through is light.
Why Pilots Care
Rapid and complete adjustment of the eye's diaphragm is required for dark adaptation; any interference delays or limits night vision and raises the of .