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Some are flared - that is, thicker at the rim than the center, like a concave lens.
有些呈喇叭形 ─ ─即边缘较中心厚, 类似凹透镜.
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A concave lens causes rays to diverge as though they are coming from a focal point.
发散透镜使光线分开,就好象从焦点上发出来的那样.
凹透镜???
凹球镜片???
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