3.(单选题,2.0分)-|||-用波长为λ的单色光垂直照射如图所示的、折射率为n2的劈形膜 ((n)_(1)gt (n)_(2),(n)_(3)gt (n)_(2)) ,观察反射光干涉、从劈形膜顶开始,第1条明条纹对应的膜厚度-|||-e [ ]-|||-n-|||-n-|||-八 dfrac (lambda )(4{n)_(2)}-|||-B dfrac (lambda )({n)_(2)}-|||-C . dfrac (lambda )(2{n)_(2)}-|||-D dfrac (3lambda )(4{n)_(2)}