几何光学：先画法线，再追光线Geometric optics: normals first, rays second
这组近年真题覆盖折射、全反射、镜面能量和视深，单题 3–7 分。常见失分点是角度基准弄错、漏算舷窗半径，以及把视深当成位移。每次先标介质与法线，再选择几何关系。These recent questions cover refraction, total internal reflection, mirror energy and apparent depth, worth 3–7 marks each. Common errors are using the wrong angle reference, omitting the aperture radius and confusing depth with displacement. Label media and normals before choosing the geometry.
1折射与全反射Refraction and total internal reflection
角度从法线量起。只有从高折射率介质射向低折射率介质，才可能发生全反射。Measure angles from the normal. Total internal reflection is possible only from a higher-index medium toward a lower-index medium.
在棱镜中再加上 r₁ + r₂ = A。For a prism also use r₁ + r₂ = A.
2折射率匹配与视深Index matching and apparent depth
折射率相等，透明界面不使光线偏折。平行多层介质中，近轴视深等于各层视厚度的和。Equal indices cause no bending at a transparent boundary. For parallel layers the paraxial apparent depth is the sum of the individual apparent thicknesses.
此视深公式假定从空气中近乎垂直观察。This depth formula assumes near-normal viewing from air.
3镜面的方向与能量Mirror direction and energy
反射角等于入射角。每次碰撞都把能量分为反射和透射两部分；相同镜面的多次碰撞可用等比级数求和。Reflection angle equals incidence angle. Each hit splits energy into reflected and transmitted portions; repeated encounters with identical mirrors form a geometric series.
能量守恒是检查级数是否漏项的好方法。Energy conservation checks whether the series misses any terms.
| 子题型Sub-type | 识别特征Clue | 核心步骤Core step | 真题Question |
|---|---|---|---|
| 全反射与视野TIR and field of view | 圆形舷窗Circular porthole | 临界角 → 半径 → 面积Critical angle → radius → area | 2025 Q1(o) |
| 镜面能量Mirror energy | 两镜多次反射Repeated reflections | 分奇偶次求和Sum odd and even hits | 2024 Q1(q) |
| 棱镜Prism | 能否从另一面出射Escape through another face | 临界角 → 几何 → SnellCritical angle → geometry → Snell | 2022 Q1(t) |
| 折射率匹配Index matching | 颗粒消失Particles disappear | 体积分数加权Volume-weighted index | 2020 Q1(h) |
| 视深Apparent depth | 多层透明介质Transparent layers | 分层求和后作差Sum depths, then subtract | 2018 Q1(j) |