The diameter of a sphere is four times the focal length of a concave mirror cut from it.

凹面镜如果是从圆球上切下来的，圆球的直径会是凹面镜焦距的四倍。

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Some old favorites are dusted off and given new life, like a wonderful giant concave mirror that plays tricks on the eyes and the ears.

CNN: San Francisco's Exploratorium opens in sparkling new home