The cornea performs most of the refraction of light entering the eye, yet we still need the crystalline lens.…

Question

The cornea performs most of the refraction of light entering the eye, yet we still need the crystalline lens. What specific function does the crystalline lens serve that the cornea cannot, and what structure in the eye controls the lens's ability to perform this function?

Accepted Answer

## Model Answer The cornea performs most of the refraction, but it has a **fixed focal length** and cannot change it. The crystalline lens provides the **finer adjustment of focal length** needed to focus objects at different distances (near or far) on the retina. This ability is called **accommodation**. The structure that controls this function is the **ciliary muscles**. When they contract, the lens becomes thicker (shorter focal length) to focus nearby objects; when they relax, the lens becomes thin (longer focal length) to focus distant objects. *Source: Chapter 10, Section 10.1 and 10.1.1* --- ## Explanation - The key distinction: cornea = fixed refraction; lens = variable/adjustable focal length. - The term **accommodation** must be mentioned — examiners specifically look for it. - **Ciliary muscles** is the required structure (not iris, not pupil — a common error). - Three marks: one for the cornea's limitation/lens's role, one for accommodation, one for ciliary muscles.

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