Question

Bi-focal lenses are required to correct

• A. astigmatism
• B. coma
• C. presbyopia
• D. myopia

Hint:

Remember the corrections for common defects:
- Myopia \(\to\) Concave lens
- Hyperopia \(\to\) Convex lens
- Presbyopia \(\to\) Bi-focal lens
- Astigmatism \(\to\) Cylindrical lens

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks which vision defect requires the use of bi-focal lenses for correction.

Step 2: Key Formula or Approach:
Bi-focal lenses contain two distinct parts with different focal lengths: the upper part is typically a concave lens for distant vision, and the lower part is a convex lens for reading or near vision.

Step 3: Detailed Explanation:

• Myopia (Nearsightedness): Corrected using a simple concave lens.

• Astigmatism: Corrected using cylindrical lenses.

• Presbyopia: This is an age-related vision defect. With age, the ciliary muscles weaken, and the crystalline lens loses its flexibility.

• As a result, the eye loses its power of accommodation. A person suffering from presbyopia often struggles to focus on nearby objects clearly, and may also have pre-existing myopia (difficulty seeing distant objects).

• To correct both near and distant vision simultaneously without changing spectacles, a bi-focal lens is used.
- The upper portion of a bi-focal lens is concave (for distance).
- The lower portion is convex (for reading).


Step 4: Final Answer:
Bi-focal lenses are required to correct presbyopia.