4相对折射率△ = 0.005，光 纤长度为2000km，试求光纤的数值孔径_。

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SQL Error: select id,body,options from ***_enewstiku where sourceid='30263' and id < 2448226 and id<>'2448226' order by id desc limit 5 SQL Error: select id,body,options from ***_enewstiku where sourceid='30263' and id > 2448226 and id<>'2448226' order by id asc limit 5