Ray Optics And Optical Instruments

To solve this question, we need to determine the correct optical aid for a person suffering from both Myopia (nearsightedness) and Hypermetropia (farsightedness).

1. Understanding the Conditions:
- Myopia: Corrected using concave lenses.
- Hypermetropia: Corrected using convex lenses.

2. When Both Conditions Are Present:
A person with both conditions needs correction for both near and distant vision. This is achieved using a bi-focal lens, which has two parts: - The upper part corrects distant vision (myopia). - The lower part corrects near vision (hypermetropia).

3. Analyzing the Options:
- Bi-convex lens: Only for hypermetropia.
- Bi-concave lens: Only for myopia.
- Bi-focal lens: Corrects both.
- Bi-focal mirror: Not used for vision correction.

Final Answer:
Option (C) Bi-focal lens is correct.

To solve this problem, we need to recall the concept of the least distance of distinct vision in human eyes.

1. Definition:
The least distance of distinct vision is the minimum distance at which the eye can see an object clearly without strain. This is also known as the near point of the eye.

2. Standard Value:
For a normal human eye, the least distance of distinct vision is taken as: 25 cm

3. Evaluating the Options:
(A) 25 m – Too large ❌
(B) 25 mm – Too small ❌
(C) 25 cm – Correct ✅
(D) 25 km – Extremely large ❌

Final Answer:
The correct option is (C) 25 cm.

To solve the problem, we need to calculate the power of a lens whose focal length is given.

1. Formula for Power of a Lens:
The power of a lens in diopters (D) is given by:

2. Substituting the Given Focal Length:
Given:

Final Answer:
The power of the lens is 4D (Option C).