What does the iris do in response to light changes?

The iris plays a crucial role in regulating the amount of light that enters the eye, primarily through its ability to contract and dilate. This process is known as the pupillary light reflex. When exposed to bright light, the muscles in the iris contract, causing the pupil to constrict (or become smaller). This reaction helps protect the retina from excessive light exposure and enhances visual acuity in bright conditions. Conversely, in low light, the iris relaxes, leading to dilation (the pupil becomes larger), which allows more light to enter the eye for improved vision in darkness.

The other options do not accurately describe the iris's primary function in response to light changes. Flattening and expanding pertain more to the lens of the eye, which adjusts to focus on objects at varying distances. Color changes in the iris do not occur in response to light changes but are determined by genetics and melanin levels. The secretion of aqueous humor is a function of the ciliary body, not the iris, and is not influenced by changes in light. Therefore, the correct understanding of the iris's function highlights its critical role in adapting to changing light conditions.