Question

What focal length should the spectacles have for a person for whom the least distance of distinct vision is 50 cm ?
1. 50 cm, concave
2. 50 cm, convex
3. 25 cm, concave 
4. 25 cm, convex 

Answer 1

Correct Answer - Option 2 : 50 cm, convex

The Correct answer is option 2.

• For a normal person, the least distance of distinct vision is 25 cm.
• But in this problem, the least distance of distinct vision is 50 cm.
• We have 1/f = 1/v - 1/u
• Here v= -50 cm
• u = -25 cm
• 1/f = -1/50 - (-1/25)
• 1/f = 1/50 
• Focal length = 50 cm
• Hence focal length should be +50 cm i.e. convex lens should be used.