Which of the following eye defects can be corrected with a bi-focal lens?

Analyze the question: We need the eye defect that is corrected with a bi-focal lens, meaning a lens with two parts, one for distant vision and one for near vision.

Explain the concept:

• Astigmatism: The cornea or lens has an uneven curvature. It is corrected with a cylindrical lens.
• Cataract: The lens becomes cloudy or opaque. It is treated by surgery, replacing the lens.
• Hypermetropia (long-sightedness): The eyeball is too short, so near objects are blurred. It is corrected with a convex lens.
• Myopia (short-sightedness): The eyeball is too long, so distant objects are blurred. It is corrected with a concave lens.
• Presbyopia: With age, the lens loses elasticity and the ciliary muscles weaken, so the eye cannot accommodate for near objects. Many people with presbyopia also need correction for distance, since they may already be myopic. A bi-focal lens serves both needs: the upper part is for distant vision and the lower part is for reading.

Solution:

1. Bi-focal lenses combine two focal lengths in one lens.
2. Only presbyopia is the age-related defect where a person needs help with near vision while keeping distance vision, or needs both corrections.
3. The other defects are each corrected with a single type of lens (convex, concave or cylindrical) or by surgery.

Answer: E. Presbyopia

Exam tip: Remember "Presby =Prestige of old age." Presbyopia comes with ageing, usually after about 40 years, and bi-focals are the classic solution. Also remember: myopia needs a concave lens, hypermetropia needs a convex lens, and astigmatism needs a cylindrical lens.