A person needs a lens of power –5.5 dioptres for correcting 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 and lens type required for correcting 1. distant vision, 2. near vision. [3 MARKS]
A person needs a lens of power –5.5 dioptres for correcting 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 and lens type required for correcting 1. distant vision, 2. near vision. [3 MARKS]
Each point: 1 Mark
1. Power of the lens used for correcting distant vision = -5.5 D
Focal length of the required lens, f = 1P
f = 1−5.5 = -0.181 m
The focal length of the lens for correcting distant vision is -0.181m
Negative sign implies that the lens required is concave.
2. Power of the lens used for correcting near vision = +1.5 D
Focal length of the required lens, f = 1P
f = 1+1.5 = +0.667 m
The focal length of the lens for correcting distant vision is +0.667 m
Positive sign implies that the lens required is convex.
Each point: 1 Mark
1. Power of the lens used for correcting distant vision = -5.5 D
Focal length of the required lens, f = 1P
f = 1−5.5 = -0.181 m
The focal length of the lens for correcting distant vision is -0.181m
Negative sign implies that the lens required is concave.
2. Power of the lens used for correcting near vision = +1.5 D
Focal length of the required lens, f = 1P
f = 1+1.5 = +0.667 m
The focal length of the lens for correcting distant vision is +0.667 m
Positive sign implies that the lens required is convex.
