Question

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 dioptres. What is the focal length of the lens required for correcting

1. distant vision  

2. near vision ?

Answer 1

1. For distant vision: 

P = -5.5D 

P = 1/f 

f = 1/P = 1/(-5.5) 

= -0.1818 m = -18.18cm 

2. For Near Vision: 

P = 1.5D P = 1/f 

f = 1/P = 1/1.5 = 0.6666 = 66.66 cm