干涉与衍射：先数光程，再判断相位Interference & diffraction: optical path and phase
本组选取 2012–2020 年的五题，共 22 分，单题 2–6 分。题库中这一主题并非每年出现；常见失分点是斜入射的符号、薄膜反射相变，以及把条纹移动数误当作级次。最后一题用浮力求膜厚，是跨主题拓展。These five questions from 2012–2020 total 22 marks, with 2–6 marks each. This topic is not present every year in this collection. Frequent traps are oblique-incidence signs, reflection phase shifts and confusing fringe shifts with orders. The final problem is a cross-topic extension using buoyancy to find film thickness.
1光程差控制明暗Optical path controls brightness
相干光有稳定的相位关系。等振幅两束光的相位差包括传播项与反射相变；只计算几何路程可能漏掉折射率。Coherent beams maintain a stable phase relation. Their relative phase includes propagation and reflection shifts; geometric distance alone omits refractive index.
无额外相变时：Δ = mλ 为亮纹，Δ = (m+½)λ 为暗纹。Without an extra phase shift: Δ = mλ is bright; Δ = (m+½)λ is dark.
2光栅：几何与级次Gratings: geometry and order
正入射时用 d sinθ = mλ；屏幕给出的通常是 tanθ，需先转成 sinθ。斜入射时必须给角度规定正负号。At normal incidence use d sinθ = mλ. Screen geometry usually gives tanθ, so convert to sinθ. At oblique incidence define signed angles.
第三束光不一定是第三级；零级也属于透射光。The third beam need not be third order; zero order is transmitted too.
3薄膜与介质管Films and gas tubes
薄膜的反射光多走一个往返，介质管通常只有单程。低折射率到高折射率的反射带来 π 相变，先逐个界面检查。A reflected film ray makes an extra round trip; a gas tube is normally crossed once. Reflection toward higher index adds π phase; check each interface.
真空波长 λ 与介质内波长 λ/n 要区分。Distinguish vacuum wavelength λ from in-medium wavelength λ/n.
| 子题型Sub-type | 识别特征Clue | 核心步骤Core step | 真题Question |
|---|---|---|---|
| 正入射光栅Normal grating | 屏距与光斑距离Screen and spot distances | 三角函数 + 光栅方程Geometry + grating equation | 2020(l) |
| 斜入射光栅Oblique grating | 相邻级次Adjacent orders | 两式相减Subtract equations | 2018(o) |
| 薄膜反射Film reflection | 两种界面Two interfaces | 相变 + 往返光程Phase shifts + round trip | 2017(n) |
| 双缝条纹移动Double-slit shift | 抽气管Evacuated tube | Nλ = (n−1)LNλ = (n−1)L | 2017(o) |
| 膜厚拓展Film extension | 氦气泡悬浮Floating helium bubble | 浮力平衡Buoyancy balance | 2012(p) |