For those fortunate souls who do not need glasses, the lens ofthe eye adjusts its focal length in order to form a proper image onthe retina. This typically means that very distant objects as wellas objects as close as 25 cm can be seen clearly.
Many of us need corrective lenses since the lens in our eye cannotadjust sufficiently to produce a clear image over the full rangeobject distances. This may be because the lens itself does notadjust well or because the eye is either longer or shorter than‘normal’.
In the case of someone who is nearsighted (can see up close) theeye may only be able to see clearly items up to 50cm or 1m away(this would be the far point). In order to see something furtheraway, a lens (either glasses or contacts) is used to produce avirtual image of a distant object at the person’s far point. Theireye can then accommodate the rest of the way and produce a clearimage.

Suppose a person who has a far point of 69.3 cm is tryingto view a distant object. What is the focal length (with correctsign) of a lens that would take a distant object and make an imageon the same side of the lens as the object a distance 69.3 cmfrom the lens?

Lenses are prescribed in terms of their refractive power, whichis expressed in terms of diopters (see the text or your favoritesearch engine for the definition of a diopter). What is therefractive power of this lens in terms of diopters?