📚 Physics Experiments: Core Optics Experiments & Problem-Solving Strategies | 物理实验：光学核心实验考点与解题思路
Optics experiments are a fundamental part of A-level Physics practical assessments. This article summarises the essential experiments, common measurement methods, and systematic approaches to solving exam-style questions.
光学实验是 A-level 物理实验考查的核心板块。本文系统梳理关键实验、常用测量方法以及应对考试题目的通用解题思路。
1. Experimental Foundations: Error Sources & Measurement Tools | 实验基础：误差来源与测量工具
Before tackling any optics experiment, identify the main sources of systematic and random errors: parallax in reading scales, finite width of light beams, and uncertainty in judging the sharpest image.
在解决任何光学实验之前，首先要识别主要系统误差和随机误差的来源：刻度读数时的视差、光束宽度有限、判断最清晰像时的不确定性。
Use a pin method to locate rays accurately. Place pins vertically, and ensure the eye aligns with two pins to determine a straight line. Measure angles from the normal, not from the surface.
使用大头针法可以准确定位光线。大头针应竖直插放，眼睛透过两枚大头针共线来确定一条直线。角度应相对法线测量，而不是相对界面测量。
For lens experiments, use a metre rule with millimetre divisions and take multiple readings to reduce random errors. Always record uncertainties and repeat measurements.
在透镜实验中，使用毫米刻度的米尺，并多次读数以减小随机误差。始终记录不确定度并重复测量。
| Source of Error | Solution |
| Parallax / 视差 | Use a mirror scale or place eye directly above the marker / 使用镜面刻度或正视标记 |
| Beam width / 光束宽度 | Use a narrow slit or thin pins / 使用窄缝或细针 |
| Image focusing / 聚焦判断 | Move screen back and forth to find minimum sharp image / 反复移动屏幕找到最小清晰像 |
2. Measuring Refractive Index Using a Glass Block | 测定玻璃砖折射率
Place a rectangular glass block on a sheet of paper. Trace its outline, then insert two pins on one side to define the incident ray. Look through the block and place two more pins that line up with the first pair, marking the emergent ray.
将矩形玻璃砖放在白纸上，描出轮廓。在一侧插入两根大头针以确定入射光线。透过玻璃砖观察，再插两根大头针使它们与前两根共线，从而标出射出的光线。
Draw the incident and emergent rays, join their intersection points to obtain the refracted ray inside the block. Measure the angle of incidence i and the angle of refraction r with respect to the normal.
画出射光线和出射光线，连接交点得到玻璃砖内部折射光线。分别测量入射角 i 和折射角 r（均相对法线）。
Calculate the refractive index using Snell’s law:
n = sin i / sin r
Repeat for different angles of incidence and plot sin i against sin r. The gradient of the straight line through the origin equals the refractive index n.
对不同入射角重复测量，绘制 sin i 与 sin r 的关系图。过原点的直线斜率即为折射率 n。
3. Total Internal Reflection and Critical Angle | 全反射与临界角
Total internal reflection occurs when light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle C.
当光从光密介质射向光疏介质，且入射角大于临界角 C 时，发生全反射。
Use a semicircular glass block. Direct a narrow ray towards the centre of the flat face from the curved side. Gradually increase the angle of incidence until the refracted ray just disappears; this is the critical angle.
使用半圆形玻璃砖。让一束细光从弧面射向平面面的圆心。逐渐增大入射角，直到折射光线刚好消失，此时的入射角即为临界角。
The relationship between critical angle and refractive index is:
sin C = 1 / n
A common exam question asks you to predict whether a ray will undergo total internal reflection. Compare the angle of incidence with C; if i > C and the light is travelling into a less dense medium, total internal reflection occurs.
常见考题要求判断光线是否会全反射。比较入射角与 C：若 i > C 且光射向光疏介质，则发生全反射。
4. Focal Length of a Convex Lens | 凸透镜焦距测定
The thin lens equation relates object distance u, image distance v, and focal length f:
1/f = 1/u + 1/v
Set up an illuminated object, a convex lens, and a screen on an optical bench. Move the screen until a sharp image is formed. Record u and v. Repeat for at least five different object distances.
在光具座上放置发光物体、凸透镜和屏幕。移动屏幕直到形成清晰像。记录物距 u 和像距 v。至少取五个不同的物距重复实验。
Plot 1/v against 1/u; the intercepts give 1/f. Alternatively, plot 1/u versus 1/v and find the intercepts where the line crosses the axes.
绘制 1/v 对 1/u 的图像，截距对应 1/f。也可以绘 1/u 对 1/v，从两轴截距求焦距。
A quick method is the autofocus method: place the object and screen a fixed distance apart (greater than 4f). Move the lens to two positions that give sharp images. The focal length is given by the displacement method:
快速方法为自准法：将物体与屏幕固定距离（大于 4f）。移动透镜到两个成清晰像的位置。位移法公式为：
f = (D² – d²) / 4D
where D is the distance between object and screen, and d is the distance between the two lens positions.
其中 D 是物体到屏幕的距离，d 是两个透镜位置之间的距离。
5. Young’s Double-Slit Interference | 杨氏双缝干涉
This experiment measures the wavelength of light by observing interference fringes from two coherent slits.
该实验通过观察两条相干缝产生的干涉条纹来测量光的波长。
Set up a laser or illuminated single slit, then a double slit, and a screen. The fringe spacing Δy is related to the wavelength λ, slit separation d, and screen-to-slit distance D by:
使用激光或单缝光源，随后放置双缝和屏幕。条纹间距 Δy 与波长 λ、双缝间距 d、缝到屏幕距离 D 的关系为：
λ = d Δy / D
For best results, use a sharp ruler to measure across several fringes (e.g. 10 fringes) and divide by the number of spacings to reduce uncertainty. Ensure the slits are perpendicular to the laser beam and the screen is parallel to the slits.
为获得最佳结果，测量多个条纹（例如 10 个条纹）的总宽度并除以间隔数，以减小不确定度。确保双缝垂直于激光束，且屏幕与双缝平行。
Common exam pitfalls: forgetting to convert units of d and D to metres, or measuring the bright spot width instead of the centre-to-centre spacing.
常见错误：忘记将 d 和 D 转换为米，或测量亮斑宽度而不是中心到中心的间距。
6. Single-Slit Diffraction | 单缝衍射
When light passes through a narrow slit, a diffraction pattern is formed with a central maximum and weaker side maxima. For a slit of width a, the minima occur at:
光通过窄缝时形成衍射图样，中央亮纹最强，两侧明纹较暗。对于缝宽 a，暗纹位置满足：
a sin θ = nλ (n = 1, 2, 3, …)
In the small-angle approximation, sin θ ≈ tan θ = x/D, where x is the distance from the centre to the first minimum and D is the slit-to-screen distance.
在小角近似下，sin θ ≈ tan θ = x/D，其中 x 是中央亮纹中心到第一暗纹的距离，D 是缝到屏幕的距离。
Key observation: increasing the slit width a makes the central maximum narrower; increasing the wavelength makes it wider.
关键结论：增大缝宽 a，中央亮纹变窄；增大波长，中央亮纹变宽。
7. Polarisation of Light | 光的偏振
Polarisation demonstrates that light is a transverse wave. Use two polarising filters; the transmitted intensity I depends on the angle θ between their transmission axes according to Malus’s law:
偏振现象证明光是横波。使用两个偏振片，透射光强 I 与两偏振片透光轴夹角 θ 有关，满足马吕斯定律：
I = I₀ cos² θ
Rotate one filter while keeping the other fixed. Measure the transmitted intensity using a light sensor. At θ = 0°, intensity is maximum; at 90°, it is zero (for ideal polarisers).
固定一个偏振片，旋转另一个偏振片，用光传感器测量透射光强。当 θ = 0° 时光强最大；θ = 90° 时为零（理想偏振片）。
Exam questions often ask why reflected light is polarised, or how to produce polarised light from unpolarised light. Mention the electric field vector oscillates perpendicular to the direction of propagation.
考题常问反射光为何偏振，或如何从自然光获得偏振光。需说明电场矢量垂直于传播方向振动。
8. Measuring the Refractive Index of a Liquid | 测量液体折射率
A common alternative experiment uses a pin inside a liquid. Place a pin at the bottom of a container, then a second pin above the surface; measure the apparent depth by parallax.
另一种常见实验使用液体中的大头针。将一根大头针放在容器底部，另一根放在液面上方；通过视差法测量视深。
The refractive index is given by:
n = real depth / apparent depth
Use a travelling microscope to focus on the pin and on its image. Take care to read the vernier scale accurately. Repeat at several depths and take an average.
使用移测显微镜分别聚焦于真实大头针和它的像。注意精确读取游标卡尺刻度。在不同深度重复测量并取平均值。
9. Experimental Design and Data Analysis | 实验设计与数据分析思路
When answering design-based questions, always state: (a) apparatus, (b) procedure, (c) variables to control, (d) how to improve accuracy.
回答设计类题目时，应明确：(a) 所需器材，(b) 实验步骤，(c) 需控制的变量，(d) 如何提高精度。
For graphs, plot the linearised form of the equation. For example, for Snell’s law plot sin i against sin r; for lens equation plot 1/u against 1/v. Include error bars and draw the line of best fit.
作图时，应将公式转化为线性形式。例如，斯涅尔定律绘 sin i 对 sin r；透镜公式绘 1/u 对 1/v。添加误差棒并绘制最佳拟合直线。
Compare the gradient with the theoretical value and comment on whether the intercept is consistent with zero. State one systematic error and one method to minimise it.
将斜率与理论值比较，并判断截距是否与零一致。说明一个系统误差及减小该误差的方法。
10. Common Exam Questions and Answer Templates | 常见考题与答题模板
Example 1: “Explain how to determine the refractive index of a transparent solid.” Use the glass block method: trace rays, measure angles, plot sin i vs sin r, gradient = n.
例 1：“说明如何测定透明固体的折射率。”使用玻璃砖法：描迹光线、测量角度、绘制 sin i 对 sin r 图，斜率即为 n。
Example 2: “A student obtains a fringe spacing of 2.1 mm with a double slit of separation 0.50 mm, screen 1.20 m away. Calculate the wavelength.” Answer: λ = dΔy/D = (0.50×10⁻³ × 2.1×10⁻³) / 1.20 = 8.75×10⁻⁷ m.
例 2：“学生测得条纹间距 2.1 mm，双缝间距 0.50 mm，屏距 1.20 m，计算波长。”解：λ = dΔy/D = (0.50×10⁻³ × 2.1×10⁻³) / 1.20 = 8.75×10⁻⁷ m。
For multiple-choice questions, remember the order of magnitude: visible light λ ≈ 500 nm = 5×10⁻⁷ m. Always check units before substituting.
对于选择题，记住可见光波长数量级：λ ≈ 500 nm = 5×10⁻⁷ m。代入数值前务必检查单位。
Published by TutorHao | Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply