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Physics · 术语表

什么是 Far point, near point？

Definition 4.2 University Physics — Year 1 · 第 4 章 — Optical Instruments and the Eye

The far point (punctum remotum) is the object point seen sharply without accommodating; the near point (punctum proximum) the closest point seen sharply with maximal accommodation. For a normal (emmetropic) eye the far point is at infinity and the near point at $d_m \approx 25\,\mathrm{cm}$ , the conventional distance of distinct vision. The difference of vergences needed for the two is the amplitude of accommodation: about $4\,\delta$ for a young adult, falling with age.

The reduced eye: a thin lens of variable focal length and a retina 17\, mm behind it. Relaxed, it focuses distant objects; accommodating, it shortens f' to bring a near object’s image onto the retina. The image is inverted — the brain turns it back. — The reduced eye: a thin lens of variable focal length and a retina $17\,\mathrm{mm}$ behind it. Relaxed, it focuses distant objects; accommodating, it shortens $f'$ to bring a near object’s image onto the retina. The image is inverted — the brain turns it back.

示例

Example 4.3 (The eye’s vergence)

Distant object: image at $d = 17\,\mathrm{mm}$ means $f' = 17\,\mathrm{mm}$ , $V = 1/0.017 = 59\,\delta$ . Object at $25\,\mathrm{cm}$ : $V = 1/0.017 + 1/0.25 = 59 + 4 = 63\,\delta$ . Four diopters of accommodation out of sixty: the crystalline lens adds only a few percent to the cornea’s job, but those few percent are everything for reading.

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