A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision?

Q. 5 :- A person needs a lens of power –5.5 dioptres for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 dioptre. What is the focal length of the lens required for correcting (i) distant vision, and (ii) near vision? 

 

Answer:-

(i) In case of distant vision

1
P =
f
1
f =
P
1
f = −
5.5
10
f = −
55
2
f = −
11
2
f = −
11
f = − 0.181 m
f = − 18.1 cm

(ii) In case of near vision.

1
P =
f
1
f =
P
1
f = −
1.5
10
f = −
15
2
f =
3
f = 0.667 m
f = 66.7 cm