Optical Instruments

Question

A nearsighted eye is corrected by placing a diverging lens in front of the eye. The lens will create a virtual image of a distant object at the far point (the farthest an object can be and still be in focus) of the myopic viewer where it will be clearly seen. In the traditional treatment of myopia, an object at infinity is focused to the far point of the eye.

If an individual has a far point of 70 cm, prescribe the correct power of the lens that is needed. 

Answer 1

If the far point is 70 cms, it means we need to create a virtual image at 70 cms for an object placed a long way away (assume infinity)

Therefore we get -1/70 + 1/∞ = 1/f

Therefore the focal length is -70 cms

The strength in diopters is -1.43