**Core Concept:** The lens of the eye is a double-layered structure composed of two main components: the outer, transparent outer layer called the "capsule" and the inner, pigmented layer called the "core." Both layers contribute to the refractive index of the lens, which is essential for focusing light and maintaining clear vision.

**Why the Correct Answer is Right:** The refractive index is determined by the density and protein content of the lens. The core has a higher protein content and density compared to the capsule, leading to a higher refractive index. This difference in refractive index between the core and capsule allows for the focusing of light within the eye, enabling clear vision.

**Why Each Wrong Option is Incorrect:**

A. The refractive index is primarily determined by the core's protein content and density, not the capsule.
B. Although the capsule is less dense than the core, its contribution to the overall refractive index is less significant due to its lower protein content.
C. The lens's refractive index is not solely determined by the capsule's refractive index, as the core also plays a crucial role.
D. The refractive index is not determined by the lens's color (pigmentation) but rather by its protein content and density.

**Clinical Pearl:** Understanding the anatomy and function of the lens is essential for understanding refractive errors, such as myopia (nearsightedness) and hyperopia (farsightedness). These conditions may require corrective lenses, such as glasses or contact lenses, to improve vision and maintain eye health.

**Correct Answer:** D. The lens's refractive index is determined by the core's protein content and density, which is higher than that of the capsule.