📚 IB OCR Science: Light Key Points | IB OCR 科学：光 考点精讲
Light is a cornerstone topic in both IB Physics and OCR A-Level Physics, covering everything from ray optics to wave phenomena. Mastering the principles of reflection, refraction, interference, and polarisation is essential for tackling both conceptual and calculation-based exam questions. This revision guide consolidates the key points, definitions, and equations you need to succeed, with a clear bilingual approach to support learners across curricula.
光是 IB 物理和 OCR A-Level 物理的核心主题，涵盖从几何光学到波动现象的一切。掌握反射、折射、干涉和偏振的基本原理，对于应对概念题和计算题都至关重要。这份考点精讲整合了需要掌握的关键点、定义和方程，并用清晰的双语方式呈现，以帮助学习不同课程的同学。
1. The Nature of Light | 光的本质
Light is an electromagnetic wave that can travel through a vacuum. It displays both wave-like properties (interference, diffraction) and particle-like properties (photoelectric effect). In IB and OCR syllabuses, the wave model is used to explain most optical phenomena encountered at this level, including reflection, refraction, and polarisation.
光是一种可以在真空中传播的电磁波。它同时表现出波动性（干涉、衍射）和粒子性（光电效应）。在 IB 和 OCR 课程中，波动力学模型用于解释现阶段遇到的大多数光学现象，包括反射、折射和偏振。
All electromagnetic waves travel at the speed of light in a vacuum, c = 3.00 × 10⁸ m s⁻¹. The relationship between speed, frequency, and wavelength is given by c = fλ. Frequency remains constant when light passes from one medium to another; speed and wavelength change.
所有电磁波在真空中都以光速 c = 3.00 × 10⁸ m s⁻¹ 传播。波速、频率和波长之间的关系由 c = fλ 给出。当光从一种介质进入另一种介质时，频率保持不变，而波速和波长会改变。
2. Reflection & Plane Mirrors | 反射与平面镜
The law of reflection states that the angle of incidence is equal to the angle of reflection, both measured from the normal. This is always true for smooth surfaces, producing specular reflection. Rough surfaces cause diffuse reflection, scattering light in many directions.
反射定律指出入射角等于反射角，两者均从法线量起。对于光滑表面，这始终成立，产生镜面反射。粗糙表面则引起漫反射，将光散射到许多方向。
In a plane mirror, the image is virtual, upright, laterally inverted, and the same size as the object. The image distance behind the mirror equals the object distance in front of it. These properties are frequently tested through ray diagrams.
平面镜所成的像是虚像、正立、左右颠倒且与物体等大。像在镜后的距离等于物体在镜前的距离。这些性质常通过光线图进行考查。
3. Refraction & Snell’s Law | 折射与斯涅尔定律
Refraction occurs when light passes from one transparent medium to another and changes speed. This change in speed causes a change in direction unless the light enters along the normal. The refractive index n of a medium is defined as n = c / v, where v is the speed of light in that medium.
当光从一种透明介质进入另一种介质并改变速度时，会发生折射。除非光沿法线入射，否则速度的改变会导致方向变化。介质的折射率 n 定义为 n = c / v，其中 v 是光在该介质中的速度。
Snell’s law relates the angles and refractive indices:
n₁ sin θ₁ = n₂ sin θ₂
斯涅尔定律将角度和折射率关联起来：
n₁ sin θ₁ = n₂ sin θ₂
When light travels from a less dense to a more dense optical medium (n₂ > n₁), it bends towards the normal. Conversely, it bends away from the normal when moving into a less dense medium. A common exam task is to calculate the unknown angle or refractive index using this equation.
当光从光疏介质射向光密介质（n₂ > n₁）时，光线向法线偏折。反之，当进入光疏介质时，光线偏离法线。利用该方程计算未知角度或折射率是常见的考试任务。
4. Total Internal Reflection | 全内反射
Total internal reflection (TIR) occurs when light travelling in a denser medium hits the boundary with a less dense medium at an angle of incidence greater than the critical angle C. At the critical angle, the angle of refraction is 90°.
当光在光密介质中传播，以大于临界角 C 的入射角射向与光疏介质的界面时，会发生全内反射（TIR）。在临界角下，折射角为 90°。
The critical angle is given by:
sin C = n₂ / n₁
其中，n₁ 是光密介质的折射率，n₂ 是光疏介质的折射率。如果光从介质射向真空或空气（n₂ ≈ 1），公式简化为：
sin C = 1 / n
The critical angle is calculated using:
sin C = n₂ / n₁
where n₁ is the refractive index of the denser medium and n₂ is that of the less dense medium. When light goes from a medium into a vacuum or air (n₂ ≈ 1), this simplifies to:
sin C = 1 / n
TIR is the principle behind optical fibres, which are widely used in communications and medical endoscopy. Both IB and OCR exams may ask you to calculate the critical angle or explain how TIR enables signal transmission with minimal loss.
全内反射是光纤背后的原理，光纤广泛用于通信和医用内窥镜。IB 和 OCR 考试都可能会要求你计算临界角，或解释全内反射如何实现低损耗的信号传输。
5. Lenses & Image Formation | 透镜与成像
Converging (convex) lenses bring parallel rays to a focus at the principal focus. Diverging (concave) lenses spread parallel rays outwards so they appear to diverge from a virtual focus. Ray diagrams must show at least two principal rays to locate the image.
会聚（凸）透镜将平行光线汇聚到主焦点。发散（凹）透镜则使平行光线散开，看似从虚焦点发散而出。光线图必须至少画出两条主光线来确定像的位置。
The thin lens equation connects focal length f, object distance u, and image distance v:
1 / f = 1 / u + 1 / v
薄透镜方程把焦距 f、物距 u 和像距 v 联系起来：
1 / f = 1 / u + 1 / v
Sign conventions must be followed consistently: in the real‑is‑positive convention, distances for real objects and real images are positive, while virtual image distances are negative. Magnification is calculated as m = v / u = hi / ho, where a negative m indicates an inverted image.
必须始终遵循符号规则：在“实为正”规则中，实物和实像的距离取正值，而虚像距离取负值。放大率计算公式为 m = v / u = hi / ho，负的 m 表示像是倒立的。
6. Dispersion & Colour | 色散与颜色
Dispersion is the splitting of white light into its constituent colours because the refractive index of a medium varies slightly with wavelength. In a prism, violet light is refracted more than red light, producing a visible spectrum.
色散是指白光被分解为组成它的各种颜色，这是因为介质的折射率随波长略有变化。在棱镜中，紫光的偏折程度比红光更大，从而产生可见光谱。
The order of colours in the visible spectrum is remembered as red, orange, yellow, green, blue, indigo, violet (ROYGBIV). Colour perception can be explained by additive colour mixing (RGB) for light or subtractive mixing (CMY) for pigments. Only the wave description is needed for dispersion; the electron energy-level explanation of spectral lines is covered in atomic physics topics.
可见光谱的颜色顺序可记为红、橙、黄、绿、蓝、靛、紫（ROYGBIV）。颜色感知可以用光的加色混合（RGB）或颜料的减色混合（CMY）来解释。对于色散，只需波动描述；光谱线的电子能级解释则属于原子物理部分。
7. Interference of Light | 光的干涉
Interference occurs when two coherent light waves superpose. Coherent sources have the same frequency and a constant phase difference. Young’s double‑slit experiment demonstrates interference by producing bright and dark fringes on a screen.
当两束相干光波叠加时，就会发生干涉。相干光源具有相同的频率和恒定的相位差。杨氏双缝实验通过屏幕上产生的明暗条纹来展示干涉现象。
The fringe spacing Δx for double‑slit interference is given by:
Δx = λ D / d
其中 λ 是波长，D 是双缝到屏幕的距离，d 是双缝间距。该公式可用于测量波长或缝距。
双缝干涉的条纹间距 Δx 由下式给出：
Δx = λ D / d
where λ is the wavelength, D is the distance from the slits to the screen, and d is the slit separation. This formula can be used to measure wavelength or slit spacing.
Constructive interference (bright fringe) occurs when the path difference is a whole number of wavelengths (nλ). Destructive interference (dark fringe) occurs when the path difference is an odd multiple of half‑wavelengths ((n + ½)λ).
当光程差为波长的整数倍（nλ）时，发生相长干涉（明条纹）。当光程差为半波长的奇数倍（(n + ½)λ）时，发生相消干涉（暗条纹）。
8. Diffraction of Light | 光的衍射
Diffraction is the spreading of waves when they pass through a narrow aperture or around an obstacle. The amount of diffraction increases as the gap width approaches the wavelength of the light. Diffraction gratings produce sharper, more widely spaced maxima than double slits.
衍射是波通过窄缝或绕过障碍物时的扩展现象。缝宽越接近光的波长，衍射越显著。与双缝相比，衍射光栅能产生更锐利、间距更大的亮纹。
For a transmission diffraction grating, maxima occur at angles θ given by:
d sin θ = n λ
其中 d 是光栅常数（相邻刻线的间距），n 是级数（0, 1, 2, …）。此方程对于计算光波长或光栅刻线数至关重要。
对于透射衍射光栅，亮纹出现的角度 θ 满足：
d sin θ = n λ
where d is the grating spacing (the distance between adjacent lines) and n is the order number (0, 1, 2, …). This equation is crucial for calculating wavelengths or the number of lines per millimetre on a grating.
The diffraction pattern of a single slit consists of a central bright maximum that is twice as wide as the subsidiary maxima. The first minimum occurs at an angle θ satisfying b sin θ = λ, where b is the slit width.
单缝衍射图样由一个中央亮条纹和两侧依次减弱的次亮条纹组成，中央亮纹宽度是次亮纹的两倍。第一级暗纹对应的角度 θ 满足 b sin θ = λ，其中 b 是缝宽。
9. Polarisation of Light | 光的偏振
Polarisation demonstrates that light is a transverse wave. In unpolarised light, the electric field oscillates in all directions perpendicular to the direction of propagation. A polarising filter transmits only the component of the wave oscillating in its transmission axis.
偏振证明了光是横波。在非偏振光中，电场在所有垂直于传播方向的方向上振荡。偏振片只让沿其透射轴方向振动的分量通过。
According to Malus’s law, when perfectly polarised light passes through a second polariser (analyser), the transmitted intensity I is related to the initial intensity I₀ by:
I = I₀ cos² θ
其中 θ 是两个偏振片透射轴之间的夹角。当 θ = 90° 时，透射光强度为零（完全消光）。
根据马吕斯定律，当完全偏振光穿过第二个偏振片（检偏器）时，透射强度 I 与初始强度 I₀ 满足：
I = I₀ cos² θ
where θ is the angle between the transmission axes of the two polarisers. When θ = 90°, the transmitted intensity is zero (complete extinction).
Polarisation by reflection occurs when reflected light is partially polarised; at Brewster’s angle, the reflected ray is completely polarised parallel to the surface. This concept is included in IB Higher Level and some OCR specifications.
反射偏振现象是指反射光部分偏振；在布儒斯特角下，反射光完全偏振且偏振方向平行于表面。这一概念包含在 IB 高水平（HL）和部分 OCR 考试内容中。
10. Electromagnetic Spectrum | 电磁波谱
Visible light occupies only a small portion of the electromagnetic spectrum, typically from about 400 nm (violet) to 700 nm (red). The full spectrum, in order of increasing frequency, includes radio waves, microwaves, infrared, visible light, ultraviolet, X‑rays, and gamma rays.
可见光仅占电磁波谱的一小部分，波长约从 400 nm（紫光）到 700 nm（红光）。整个波谱按频率递增的顺序包括：无线电波、微波、红外线、可见光、紫外线、X 射线和伽马射线。
| Region 波段 | Wavelength Range 波长范围 | Key Feature 主要特征 |
|---|---|---|
| Radio waves | > 0.1 m | Communications, broadcasting |
| Microwaves | 1 mm – 0.1 m | Radar, cooking, satellite links |
| Infrared | 700 nm – 1 mm | Thermal radiation, remote controls |
| Visible light | 400 – 700 nm | Human vision, photography |
| Ultraviolet | 10 – 400 nm | Fluorescence, sterilisation |
| X‑rays | 10⁻¹⁰ – 10⁻⁸ m | Medical imaging, crystallography |
| Gamma rays | < 10⁻¹¹ m | Nuclear decay, cancer therapy |
The energy of a photon is calculated using E = h f = h c / λ, where h is Planck’s constant. This equation bridges the wave and particle models and is essential for explaining the properties of different EM bands, especially in IB questions on the photoelectric effect and atomic transitions.
光子能量用 E = h f = h c / λ 计算，其中 h 是普朗克常数。这个方程将波动模型和粒子模型联系起来，对于解释不同电磁波段的性质至关重要，尤其在 IB 中有关光电效应和原子跃迁的问题中。
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