Grades 3–5 · Organisms
Why does the black circle in your eye change size?
The dark circle in the center of the colored iris is the pupil. It is an opening, rather than a black button sitting on the eye. Light can pass through it toward the lens and retina.
In the diagramThe iris changes an opening called the pupilDiameter: 4 mmColored irisPupil openingArea ratio: 4Compared with 2 mmMuscles in the iris change the pupil opening.
Muscles in the iris change the size of this opening. In brighter surroundings the pupil usually becomes smaller; in dimmer surroundings it usually becomes larger. Other signals can also affect pupil size.
A wider circular opening has more area. In the supplied drawing, changing the diameter from 2 to 4 millimeters makes the opening’s area four times as large. That compares the openings under the same incoming illumination; it does not mean the world must look four times as bright.
The retina and brain also adjust to light. Pupil size is only one part of seeing in bright and dim surroundings. These example diameters explain the geometry, without predicting any person’s response.
About this explanation
Starting knowledge: Know that organs have parts that work together.
Sources
- National Eye Institute — How the Eyes Work