Question

Alyssa has always been mildly nearsighted, but as she’s gotten older, her eyes have lost some of their ability to accommodate to focus on close objects. She now has a far point of 2 m and a near point of 1 m; Alyssa needs a prescription for bifocals. The outer rim of her lenses will have a refractive power that will allow her to focus on distant objects; the center of her lenses will have a different refractive power that will allow her to focus on close objects. What two lens strengths, in diopters, should be prescribed to restore “normal” vision for Alyssa?

Answer 1

Focal length for the lens used for correction of seeing distant object will be :

1/f= 1/v - 1/u

Far point is at 2m which was supposed to be infinity for a healthy eye. Now he can see anything clearly if kept at 2m. So we want the lens to make the image at 2m of objects kept at infinity.

1/f = - 1/2 - (-1/infinity)

Since power = 1/f so the power will be - 0.5D for outer rim of glass.

Similarly for a healthy eye near point is 25 cm while here it is1m. So the lens is needed to create an image at 1m if object is at 25 cm

Power=1/f= (- 1/1)- (-1/0.25)

Power = +3D of converging lens for focus on close object is used in center