The correct option is **B. Astigmatism**. ### Detailed Explanation: 1. **Understanding Astigmatism**: - Astigmatism is a common refractive error caused by an irregular shape of the cornea or lens of the eye. Instead of being perfectly spherical, the cornea or lens may be more curved in one direction than the other, leading to blurred or distorted vision at all distances. - This irregular curvature means that light rays entering the eye do not converge at a single focal point on the retina, resulting in multiple focal points and thus causing visual distortion. 2. **Role of Cylindrical Lenses**: - A cylindrical lens is specifically designed to correct astigmatism. It has different optical power in different meridians (the vertical and horizontal planes). This means that it can bend light more in one direction than the other, compensating for the uneven curvature of the cornea or lens. - When a cylindrical lens is placed in front of the eye, it helps to focus the light rays correctly onto the retina, allowing for clearer vision. 3. **Why Other Options Are Incorrect**: - **A. Myopia**: Myopia, or nearsightedness, occurs when the eye is too long or the cornea is too curved, causing distant objects to appear blurry. Myopia is typically corrected with concave lenses (negative power lenses), not cylindrical lenses. - **C. Presbyopia**: Presbyopia is an age-related condition where the eye's lens loses flexibility, making it difficult to focus on close objects. This condition is usually corrected with bifocal or multifocal lenses, which have different optical powers for distance and near vision, rather than cylindrical lenses. - **D. Chromatic Aberration**: Chromatic aberration is an optical phenomenon that occurs when a lens fails to focus all colors to the same convergence point. It is not an eye defect but rather a lens-related issue. It cannot be corrected with cylindrical lenses; instead, it requires specially designed lenses or coatings to minimize the effect. ### Summary of Key Points: - **Astigmatism** is caused by an irregularly shaped cornea or lens, leading to distorted vision. - **Cylindrical lenses** correct astigmatism by having different optical powers in different meridians, allowing light to focus correctly on the retina. - **Myopia** is corrected with concave lenses, **presbyopia** with bifocal or multifocal lenses, and **chromatic aberration** is a lens issue, not an eye defect. ### Revision Summary: - Astigmatism is corrected with cylindrical lenses due to irregular corneal shape. - Myopia requires concave lenses; presbyopia needs bifocal/multifocal lenses. - Chromatic aberration is not an eye defect but a lens issue, unrelated to cylindrical lenses.