📚 A-Level OCR Science: Light Key Points | A-Level OCR 科学：光 考点精讲
Light is one of the most fascinating and central topics in the OCR A-Level Physics course, bridging classical optics and modern quantum physics. This article provides a concise yet thorough revision of the key concepts, formulas, and exam tips you need to master the behaviour of light.
光在 OCR A-Level 物理课程中是最迷人和最核心的主题之一，它连接了经典光学与现代量子物理。本文为你提供简明而透彻的考点精讲，涵盖关键概念、公式及应试要诀，助你彻底掌握光的各种行为。
1. The Nature of Light | 光的本质
Historically, light was debated as either a wave (Huygens) or a stream of particles (Newton). Today we understand light as an electromagnetic wave that can also exhibit particle-like behaviour (photons).
历史上，人们对光究竟是波（惠更斯）还是粒子流（牛顿）争论不休。如今我们认识到光既是电磁波，也能表现出粒子般的行为（光子）。
In a vacuum, all electromagnetic waves travel at c = 3.00 × 10⁸ m s⁻¹, and visible light occupies the wavelength range roughly from 400 nm (violet) to 700 nm (red).
在真空中，所有电磁波都以 c = 3.00 × 10⁸ m s⁻¹ 传播，而可见光占据的波长范围大致为 400 nm（紫）到 700 nm（红）。
2. Reflection and Refraction | 反射与折射
The law of reflection states that the angle of incidence equals the angle of reflection (θᵢ = θᵣ), both measured from the normal.
反射定律指出，入射角等于反射角（θᵢ = θᵣ），两者均从法线量起。
When light passes from one medium to another, it changes speed, causing refraction. Snell’s law describes this quantitatively:
当光从一种介质进入另一种介质时，速度改变，引起折射。斯涅尔定律定量描述了这一关系：
n₁ sinθ₁ = n₂ sinθ₂
where n is the refractive index of each medium. The refractive index is defined as n = c/v, so a higher n means light travels slower in the medium.
其中 n 是各介质的折射率。折射率定义为 n = c/v，因此 n 越大，光在介质中传播越慢。
3. Total Internal Reflection | 全内反射
When light travels from a denser to a less dense medium (n₁ > n₂), if the angle of incidence exceeds the critical angle θc, total internal reflection occurs. The critical angle is given by:
当光从光密介质射向光疏介质（n₁ > n₂）时，若入射角大于临界角 θc，就会发生全内反射。临界角的计算式为：
sinθc = n₂ / n₁
This principle is used in optical fibres, where the core has a higher refractive index than the cladding, allowing light to be guided with minimal loss.
这一原理应用于光纤，纤芯的折射率高于包层，使得光能以极低的损耗被引导传输。
4. Lenses and the Thin Lens Equation | 透镜与薄透镜方程
A converging (convex) lens can bring parallel rays to a focus at the focal point. The distance from the lens centre to the focal point is the focal length f.
会聚（凸）透镜能将平行光线汇聚在焦点。透镜中心到焦点的距离即为焦距 f。
The thin lens equation relates object distance u, image distance v, and focal length f:
薄透镜方程联系了物距 u、像距 v 和焦距 f：
1/f = 1/u + 1/v
Sign conventions: for a real object, u is positive; for a real image, v is positive; for a convex lens, f is positive. Magnification M = v/u = image height / object height.
符号规则：实物 u 为正；实像 v 为正；凸透镜 f 为正。放大率 M = v/u = 像高 / 物高。
5. Interference and Young’s Double Slit | 干涉与杨氏双缝
Coherent light passing through two closely spaced slits produces an interference pattern of bright and dark fringes on a screen. Constructive interference occurs when the path difference is an integer multiple of the wavelength (nλ); destructive when it is an odd multiple of half wavelengths ((n+½)λ).
相干光通过两条紧邻的狭缝后，在屏幕上产生明暗相间的干涉条纹。当光程差为波长的整数倍 (nλ) 时出现相长干涉；为半波长的奇数倍 ((n+½)λ) 时出现相消干涉。
The fringe spacing x is related to the slit separation a, the screen distance D, and the wavelength λ by:
条纹间距 x 与缝间距 a、屏幕距离 D 及波长 λ 的关系为：
λ = ax / D
This experiment provides evidence for the wave nature of light and allows measurement of wavelength.
此实验为光的波动性提供了证据，并可用于测量波长。
6. Diffraction Gratings | 衍射光栅
A diffraction grating consists of many equally spaced slits. The condition for a bright fringe (maximum) is:
衍射光栅由许多等间距的狭缝组成。产生明条纹（主极大）的条件是：
d sinθ = nλ
where d is the grating spacing (1/number of lines per metre), n is the order (0, 1, 2, …), and θ is the angle from the normal.
其中 d 为光栅常数（1/每米线数），n 为级次 (0, 1, 2, …)，θ 为偏离法线的角。
Gratings produce much sharper and well-separated maxima than double slits, making them ideal for accurate wavelength determination and spectroscopy.
相比于双缝，光栅能产生更锐利、分得更开的极大，非常适合精确测定波长和光谱分析。
7. Single-Slit Diffraction | 单缝衍射
When light passes through a single narrow slit of width a, it spreads out and forms a central bright maximum flanked by dimmer fringes. The first minimum occurs when:
当光通过宽度为 a 的狭缝时，会扩散并在屏幕上形成中央明纹，两侧伴随较暗的条纹。第一级极小出现在：
a sinθ = λ
The width of the central maximum is proportional to λ/a, so longer wavelengths and narrower slits give broader central fringes.
中央明纹的宽度正比于 λ/a，因此波长越长或缝越窄，中央明纹越宽。
8. Polarisation of Light | 光的偏振
Polarisation is a phenomenon unique to transverse waves. Light from the Sun or a lamp is unpolarised (vibrations in all planes). A polarising filter transmits only the component of the electric field parallel to its transmission axis.
偏振是横波独有的现象。来自太阳或灯的光是非偏振的（在所有平面上振动）。偏振片只允许平行于其透振轴的电矢量分量通过。
When two polarisers are placed with their axes at an angle θ, the intensity of transmitted light follows Malus’s law:
当两个偏振片透振轴夹角为 θ 时，透射光强遵循马吕斯定律：
I = I₀ cos²θ
Polarisation provides strong evidence for the transverse wave model of light and has practical applications in LCD screens, sunglasses, and stress analysis.
偏振为光的横波模型提供了有力证据，并广泛应用于液晶显示屏、太阳镜和应力分析中。
9. Electromagnetic Spectrum | 电磁波谱
Visible light is only a tiny part of the full electromagnetic spectrum. All EM waves share the same speed in vacuum c, and are characterised by their frequency f and wavelength λ, related by:
可见光仅是完整电磁波谱的极小一部分。所有电磁波在真空中速度同为 c，并由频率 f 和波长 λ 表征，关系式为：
c = f λ
The spectrum, in order of increasing frequency (decreasing wavelength), includes radio, microwave, infrared, visible, ultraviolet, X-ray, gamma ray. OCR exam questions often ask you to compare properties or energies of different regions.
按频率增大（波长减小）的顺序，波谱包括无线电波、微波、红外线、可见光、紫外线、X 射线、伽马射线。OCR 考题常要求比较不同波段的性质或能量。
10. Photoelectric Effect | 光电效应
The photoelectric effect provided crucial evidence for the particle nature of light. When light of frequency f is shone on a metal surface, electrons are emitted only if f exceeds the threshold frequency f₀. The maximum kinetic energy of emitted electrons is:
光电效应为光的粒子性提供了关键证据。当频率为 f 的光照射金属表面时，仅当 f 大于截止频率 f₀ 才会发射电子。发射电子的最大动能为：
Ek max = hf − Φ
where h is Planck’s constant and Φ is the work function of the metal (Φ = h f₀). The key observation is that kinetic energy depends on frequency, not intensity, contradicting classical wave theory.
其中 h 为普朗克常数，Φ 为金属的逸出功（Φ = h f₀）。关键的实验事实是，动能取决于频率而非光强，这与经典波动理论相矛盾。
11. Wave-Particle Duality | 波粒二象性
Light exhibits both wave and particle properties. Electrons and other matter can also show wave-like behaviour, with a de Broglie wavelength given by:
光同时展现波和粒子的性质。电子等实物粒子也能表现出波动性，其德布罗意波长为：
λ = h / p = h / (mv)
This duality is central to quantum physics. Electron diffraction experiments confirm that particles have wavelengths matching the de Broglie prediction, underscoring the profound unity of nature at the atomic scale.
这一二象性是量子物理的核心。电子衍射实验证实粒子具有与德布罗意预测相符的波长，凸显了原子尺度上自然的深刻统一。
Published by TutorHao | Physics Revision Series | aleveler.com
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