For a person suffering from the eye defect called astigmatism, which type of lens is commonly prescribed to correct the vision?

Introduction / Context:
Astigmatism is a common eye defect in which the cornea or lens of the eye is not perfectly spherical. Because of this irregular curvature, light rays do not focus to a single sharp point on the retina, causing blurred or distorted vision at all distances. This question asks which type of corrective lens is used in spectacles to compensate for this uneven focusing and restore clear vision.

Given Data / Assumptions:
- The defect mentioned is astigmatism, not myopia or hypermetropia.
- Options include convex lens, bifocal lens, concave lens and cylindrical lens.
- We assume standard prescriptions used in ophthalmology for regular astigmatism.

Concept / Approach:
In astigmatism, the eye has different focal powers in different meridians (directions), often likened to a rugby ball shape rather than a perfect sphere. To correct this, lenses that have different curvatures in different directions are used. Cylindrical lenses have curvature in one direction only and no power in the perpendicular direction, allowing selective correction along a specific axis. Concave and convex spherical lenses have the same power in all directions and are used for myopia and hypermetropia respectively. Bifocal lenses combine two powers for distance and near vision but do not specifically correct astigmatic curvature unless they incorporate cylindrical components.

Step-by-Step Solution:
Step 1: Recognise that astigmatism is caused by unequal curvature of the cornea or lens in different directions. Step 2: Understand that correction requires a lens whose focusing power varies with direction. Step 3: Cylindrical lenses provide power in one meridian and no power in the perpendicular meridian, matching the requirement. Step 4: Convex and concave spherical lenses have equal power in every direction and are not sufficient alone to correct pure astigmatism. Step 5: Therefore, the correct answer is cylindrical lens.

Verification / Alternative check:
Eye prescriptions for astigmatism usually list a cylinder value (for example, CYL) and an axis specifying the orientation of the cylindrical lens. This is strong evidence that cylindrical lenses are used in practice. Standard textbooks in optics and biology also state that astigmatism is corrected by appropriate cylindrical lenses oriented along a chosen axis.

Why Other Options Are Wrong:
Convex lens: Corrects hypermetropia (long-sightedness) by converging light rays but has equal curvature in all directions, so it does not correct the directional focusing error of astigmatism by itself.
Bifocal lens: Designed to provide two different powers for near and distant vision, usually combining convex and concave components, but astigmatism correction specifically requires cylindrical power.
Concave lens: Used to correct myopia (short-sightedness) by diverging light rays uniformly; it does not address the uneven curvature characteristic of astigmatism.

Common Pitfalls:
Students often memorise that concave lenses are for myopia and convex lenses are for hypermetropia but forget about astigmatism as a separate defect. Another pitfall is to think that bifocal lenses solve all eye problems, whereas they are mainly for presbyopia. Remember that any reference to unequal focusing in different planes of the eye should prompt you to think of cylindrical lenses.

Final Answer:
The defect of astigmatism is commonly corrected using a cylindrical lens oriented at the appropriate axis.