Question

What focal length should be the reading glasses of a person be for whom the least distance of distinct vision is 50 cm? ( The distance of normal vision is 25 cm)

Answer 1

Image of the object should form beyond 50 cm than only he can see object clearly so for object at 25 cm image should form at 50cm or more than 50 cm where 'u' is the normal vision distance.
u=−25cm
v=−50cm
1/f=1/v−1/u
1/f=−1/50+1/25
1/f=1/50
so f=50cm
Hence a convex lens of focal length 50 cm should be used.
Answer-convex lens of focal length 50 cm we have to use