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【单选题】
用波长为λ的单色光垂直照射到空气劈尖上，从反射光中观察干涉条纹，距顶点L处是暗纹。使劈尖角θ连续增大，直到该处再次比现暗纹时(见图)，劈尖角的改变量△θ是（ ）。A.
λ／(2L)
B.
λ／L
C.
2λ／I
D.
λ／(4L)
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B.
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A.
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B.
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C.
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D.
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E.
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A.
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B.
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C.
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