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【简答题】
一束平行光垂直入射到某个光栅上，该光束中包含有两种波长的光：λ 1 =440nm和λ 2 =660nm．实验发现，两种波长的谱线(不计中央)第二次重合于衍射角ψ=60°的，求此光栅的光栅常数．
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