The correct answer is: b. A concave lens.

Detailed Explanation

• Option a: A convex lens
  A convex lens is used to correct hypermetropia (farsightedness), not myopia. In hypermetropia, the eyeball is too short or the lens has insufficient converging power, causing the image to form behind the retina. A convex lens helps converge light rays before they enter the eye, bringing the image onto the retina. This option is incorrect for myopia.​

• Option b: A concave lens
  Myopia (nearsightedness) occurs when the eyeball is too long or the lens is too curved, causing light rays from distant objects to focus in front of the retina. A concave (diverging) lens is used to correct this by diverging the incoming light rays so that the eye lens can focus them correctly on the retina. This option is correct.​

• Option c: Either concave or convex as long as it has the right focal length
  The shape of the lens matters, not just the focal length. Only a concave lens can correct myopia because it diverges light rays, which is necessary for this condition. Convex lenses converge light, which would worsen myopia. This option is incorrect.​

• Option d: A lens that is convex on one side and concave on the other
  This describes a meniscus lens, which can be either converging or diverging depending on the curvature. However, for myopia, only a diverging (concave) lens is effective. Such a lens would only be used if it acts as a diverging lens, not simply because it has both convex and concave surfaces. This option is misleading and incorrect for standard myopia correction.​

Introduction

Myopia, commonly known as nearsightedness, is a vision defect where distant objects appear blurry due to the eyeball being too long. The condition can be corrected effectively with a concave lens, which diverges incoming light rays, allowing the eye to focus images correctly on the retina. This article explains why concave lenses are the right choice for myopia correction and why other lens types are not suitable.

Why Concave Lenses Correct Myopia

Myopia occurs when the eyeball elongates, causing light rays from distant objects to converge in front of the retina. A concave lens diverges these rays before they enter the eye, ensuring the image is formed directly on the retina for clear vision. Convex lenses, on the other hand, converge light and are used for farsightedness (hypermetropia), not myopia.​

Common Misconceptions

Some believe any lens with the right focal length can correct myopia, but the lens shape is crucial. Only a diverging (concave) lens can address the optical error in myopia. Meniscus lenses, which have both convex and concave surfaces, are not standard for myopia correction unless they act as diverging lenses.​

Conclusion

For effective myopia correction, a concave lens is essential. It diverges light rays, enabling the eye to focus distant objects correctly. Understanding this principle helps in choosing the right corrective lenses for clear vision.​