本题目来源于试卷: Optical Instruments，类别为 IB物理学
[问答题]
Why are bifocals needed mainll.bm+o eld7. **h5vofgrow : ay bp7o3mm ;1 /nq 9ojrfoqy older persons and not generally by younger peoo o/o q7rjfpq; mm319nple?
参考答案:
As people get older, their eyes can no longer accommodate as well. It becomes harder for the muscles to change the shape of the lens, since the lens becomes less flexible with age. As people get older, their near point increases and becomes greater than the ideal value of 25 cm. They may still need the “regular” upper portion of their lenses so they can have a far point at infinity (most people need these at much younger ages), but now an older person will need a bifocal in the lower portion of their lenses to create a near point back at 25 cm for seeing close objects. Thus, as people get older, their far point is too small and their near point is too large. Bifocals can correct both of these problems.
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