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Optical Cable and information on road, the world distance shortened.
光电缆和信息路，世界距离在缩短。
youdao
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Focal length is the distance between optical centre and principal focus.
焦距就是镜片中心和焦点之间的距离。
youdao
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Focal length is the distance between optical centre and principal focus.
焦距就是镜片中心和焦点之间的距离。
youdao