Question

Contact lenses are placed on the eyeball, so the distance from the eye to an object (or an image) is the same as the distance from the lens to the object (or the image). If a person can see distant objects well, but has a nearpoint of 53.0 cm instead of the usual 25.0 cm, what power of contact lenses are needed (in diopters)?

The same person now wants to wear glasses instead; they are 1.90 cm in front of the eye. What is the power of the glasses (in diopters)?

Answer 1

To find P1
1/f = -1/53 + 1/25
1/f= 28/1325
P = 1/f = 0.021132 [cm]^(-1)
  dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres).

Therefore

P = 0.021132 x 100 diptre or diopter.

P = 2.1 dioptre.

To find P2, you need to subtract 1.9 cm from nearpoint and 25 cm.
1/f = -1/51.1 + 1/23.1
1/f= 0.02372
P = 1/f = 0.02372 [cm]^(-1)
  dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (that is, 1/metres).

Therefore

P = 0.02372 x 100 diptre or diopter.

P = 2.4 dioptre.