**Question:** Maximum refraction takes place between

A. cornea
B. cornea and lens
C. cornea and iris
D. lens

**Correct Answer:** D. lens

**Core Concept:**
Refraction is the process by which light changes direction when passing through an interface between two media with different refractive indices. In the human eye, refraction primarily occurs due to the curvature of the lens. The lens is responsible for adjusting the focal length to focus light on the retina, enabling clear vision.

**Why the Correct Answer is Right:**
The lens is an essential component of the eye responsible for focusing light onto the retina. Its refractive properties allow it to change its curvature, thus adjusting the focal length. This change in focal length is known as accommodation, which enables the eye to focus on near and far objects.

**Why Each Wrong Option is Incorrect:**
A. Cornea: Although the cornea also contributes to refraction by bending light, it does so by changing the angle of incidence, not the focal length. The lens is responsible for accommodative refraction.
B. Cornea and lens: While both cornea and lens contribute to refraction, the primary refracting element is the lens, which changes its curvature to adjust focus. The cornea primarily determines the initial angle of light entering the eye.
C. Cornea and iris: The iris controls the size of the pupil, regulating the amount of light entering the eye. Its relation to refraction is indirect, unlike the lens, which directly affects the focal length.
D. Lens: As mentioned above, the lens plays a crucial role in refraction by changing its curvature to adjust the focal length. This change in curvature enables the eye to focus on objects at different distances.

**Clinical Pearl:**
Understanding the role of the lens in refraction is essential for maintaining clear vision. By understanding the correct answer, students can appreciate the importance of accommodative responses in maintaining clear vision, particularly during near work or at varying distances.