📚 A-Level Edexcel Physics: Light Revision | A-Level Edexcel 物理：光 考点精讲
Light is one of the most fascinating topics in A-Level Physics, bridging ray optics, wave phenomena, and the particle model. In the Edexcel specification, you are expected to describe, explain, and apply principles ranging from simple reflection to the photoelectric effect, using both qualitative explanations and key equations. This revision guide highlights every essential concept, experiment, and formula you need for your exams.
光是A-Level物理中最引人入胜的主题之一，连接了几何光学、波动现象和粒子模型。在Edexcel考试大纲中，你需要描述、解释并应用从简单的反射到光电效应的各种原理，既要给出定性解释，也要使用关键公式。本复习指南将突出你应试所需的每一个基本概念、实验和方程式。
1. The Nature of Light | 光的本质
Light is an electromagnetic wave that exhibits transverse behaviour, with oscillating electric and magnetic fields perpendicular to the direction of propagation. Visible light occupies only a tiny portion of the electromagnetic spectrum, with wavelengths ranging approximately from 400 nm to 700 nm. Historically, Newton proposed a corpuscular theory, while Huygens argued for a wave model. Modern physics accepts that light has a dual nature: it produces interference and diffraction patterns like a wave, yet interacts with matter as discrete packets of energy called photons in the photoelectric effect.
光是一种电磁波，表现出横波行为，振荡的电场和磁场垂直于传播方向。可见光只占电磁波谱的极小部分，波长大约在 400 nm 到 700 nm 之间。历史上，牛顿提出微粒说，而惠更斯主张波动模型。现代物理学公认光具有二象性：它能像波一样产生干涉和衍射图样，又在光电效应中像光子一样以离散的能量包与物质相互作用。
2. The Law of Reflection | 反射定律
The law of reflection is straightforward: the angle of incidence equals the angle of reflection, and the incident ray, reflected ray, and normal to the surface all lie in the same plane. When light reflects off a smooth surface, specular (mirror-like) reflection occurs, forming a clear image. A rough surface causes diffuse reflection, scattering light in many directions and preventing image formation. In the laboratory, a ray box, plane mirror, and protractor are used to verify this law by measuring incident and reflected angles.
反射定律简洁明了：入射角等于反射角，且入射光线、反射光线和法线位于同一平面内。当光从光滑表面反射时，发生镜面反射，形成清晰的像。粗糙表面则引起漫反射，使光线向各个方向散射，无法成像。在实验室中，常用光线盒、平面镜和量角器通过测量入射角和反射角来验证该定律。
3. Refraction and Snell’s Law | 折射与斯内尔定律
Refraction occurs when light crosses a boundary between two media at an angle, changing speed and direction. Snell’s law relates the angles of incidence and refraction to the refractive indices:
n₁ sin θ₁ = n₂ sin θ₂
. The absolute refractive index of a material is n = c / v, where c is the speed of light in a vacuum and v is the speed in the medium. A classic experiment to determine the refractive index of glass uses a rectangular block, pins to trace the ray, and a graph of sin i against sin r, whose gradient gives the index.
当光以一定角度穿过两种介质的界面时，会发生折射，速度和方向均改变。斯内尔定律将入射角和折射角与折射率联系起来：
n₁ sin θ₁ = n₂ sin θ₂
。材料的绝对折射率为 n = c / v，其中 c 为真空中光速，v 为介质中的光速。测定玻璃折射率的经典实验使用矩形玻璃块、插针法追踪光线，并绘制 sin i 对 sin r 的图像，其斜率即为折射率。
4. Total Internal Reflection and Critical Angle | 全内反射与临界角
When light travels from a denser to a less dense medium (e.g., from glass to air) and the angle of incidence exceeds the critical angle θc, total internal reflection (TIR) occurs. The critical angle is found from
sin θc = n₂ / n₁
with n₁ > n₂. TIR is the operating principle behind optical fibres, which guide light along a thin core by repeated total internal reflection at the cladding boundary. Applications include high‑speed telecommunications and medical endoscopes for internal imaging.
当光从光密介质射向光疏介质（例如从玻璃到空气），且入射角超过临界角 θc 时，就会发生全内反射。临界角公式为
sin θc = n₂ / n₁
（n₁ > n₂）。全内反射是光纤工作的基本原理，光在纤芯与包层的边界上反复发生全内反射而被引导。应用包括高速通信和用于内部成像的医用内窥镜。
5. Young’s Double‑Slit Interference | 杨氏双缝干涉
Young’s interference experiment provides solid evidence for the wave nature of light. Two coherent, monochromatic slits produce a pattern of alternating bright and dark fringes on a distant screen. The fringe spacing w (the distance between adjacent bright fringes) is given by
w = λD / s
where λ is the wavelength, D is the slit‑to‑screen distance, and s is the separation of the slits. By measuring w, D, and s, the wavelength of light can be determined. Using white light yields a central white fringe flanked by coloured spectra because each wavelength interferes constructively at a slightly different angle.
杨氏干涉实验为光的波动性提供了有力证据。两个相干的单色狭缝在远处的屏幕上产生明暗相间的条纹图案。条纹间距 w（相邻亮纹中心间距）的公式为
w = λD / s
，其中 λ 为波长，D 为缝到屏幕的距离，s 为双缝间距。通过测量 w、D 和 s，可以求出光波波长。若使用白光，中央为白色条纹，两侧呈现彩色光谱，因为各波长的光在不同角度发生相长干涉。
6. The Diffraction Grating | 衍射光栅
A diffraction grating contains a large number of equally spaced parallel slits. It produces much sharper and brighter maxima than a double slit, because the light from many slits reinforces at well‑defined angles. The grating equation is
d sin θ = nλ
where d is the slit spacing (1/lines per metre), θ is the angle of the n‑th order maximum, n is an integer (0,1,2,…), and λ is the wavelength. Gratings are used in spectrometers to analyse the light from stars, identify chemical elements, and study atomic spectra.
衍射光栅含有大量等间距的平行狭缝。与双缝相比，它能产生更尖锐、更明亮的极大，因为来自多条狭缝的光在严格确定的角度上加强。光栅方程为
d sin θ = nλ
，其中 d 为缝距（线数/米的倒数），θ 为第 n 级极大的角度，n 为整数（0,1,2,…），λ 为波长。光栅用于光谱仪中分析星光、鉴别化学元素和研究原子光谱。
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 diffraction pattern. The central maximum is bright and wide, surrounded by much dimmer secondary maxima. Minima (dark fringes) occur at angles that satisfy
a sin θ = nλ
where n = 1,2,3,… . The intensity falls off rapidly away from the centre. This behaviour is a hallmark of wave optics and explains the inevitable blurring that limits the resolution of telescopes and microscopes.
当光通过宽度为 a 的狭窄单缝时，光会散开并形成衍射图样。中央极大明亮而宽阔，两侧分布着暗得多的次级极大。极小（暗纹）发生在满足
a sin θ = nλ
的角度上，其中 n = 1,2,3,… 。强度从中心向外迅速下降。这一行为是波动光学的标志，也解释了限制望远镜和显微镜分辨率的必然模糊现象。
8. Polarisation of Light | 光的偏振
Polarisation demonstrates unequivocally that light is a transverse wave. Unpolarised light from a filament lamp vibrates in all planes perpendicular to its travel direction. A polarising filter transmits only the components oscillating in one specific direction. If a second ‘analyser’ filter is rotated by an angle θ relative to the first, the transmitted intensity follows Malus’s law:
I = I₀ cos² θ
. Polaroid sunglasses take advantage of this to reduce glare from reflections, and liquid‑crystal displays (LCDs) rely on polarisation control to produce images.
偏振清晰地证明光是横波。来自白炽灯的非偏振光在所有垂直于传播方向的平面内振动。偏振滤光片只允许沿某一特定方向振动的分量通过。若第二片‘检偏器’相对于第一片旋转角度 θ，透射光强遵从马吕斯定律：
I = I₀ cos² θ
。偏振太阳镜利用此原理减少反射眩光，液晶显示屏则依靠偏振控制来产生图像。
9. The Photoelectric Effect | 光电效应
The photoelectric effect is the emission of electrons from a metal surface when light of sufficiently high frequency shines on it. Key observations include: electrons are emitted almost instantaneously; a threshold frequency f₀ exists below which no electrons are emitted, no matter how intense the light; the maximum kinetic energy of emitted electrons depends only on frequency, not on intensity. These facts cannot be explained by the wave model but are perfectly described by Einstein’s photon theory. The energy of a photon is E = hf. The photoelectric equation is
Eₖ max = hf − φ
where φ is the work function of the metal. The stopping potential Vₛ is related to the maximum kinetic energy by e Vₛ = Eₖ max. A typical experiment illuminates a photocell with light of various frequencies and measures the stopping potential to determine Planck’s constant h and the work function.
光电效应是指当频率足够高的光照射在金属表面时，电子从表面逸出的现象。关键观察结果包括：电子几乎瞬间逸出；存在截止频率 f₀，低于该频率，无论光强多大都没有电子逸出；逸出电子的最大动能只依赖于频率，与光强无关。这些事实无法用波动模型解释，但爱因斯坦的光子理论却能完美描述。光子能量为 E = hf。光电方程为
Eₖ max = hf − φ
，其中 φ 是金属的功函数。遏止电压 Vₛ 与最大动能的关系为 e Vₛ = Eₖ max。典型的实验使用不同频率的光照射光电管，测量遏止电压，从而测定普朗克常量 h 和功函数。
The following table summarises which model of light successfully accounts for key phenomena. The wave model explains interference, diffraction, and polarisation, whereas the particle model is required for the photoelectric effect.
下表总结了哪种光模型能成功解释关键现象。波动模型能解释干涉、衍射和偏振，而光电效应则需要粒子模型。
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