A person needs a lens of power, – 5.5 dioptres for correcting his di

1.	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 correcting1. distant vision 2. near vision ?
Answer» 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

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 correcting1. distant vision 2. near vision ?

Answer»

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