📚 Refraction of Light | 光的折射
Refraction is the bending of light as it passes from one transparent medium to another of different density. This phenomenon is fundamental to understanding how lenses work, why a straw looks bent in a glass of water, and how optical fibres transmit information. In the CCEA GCSE Physics specification, you need to be able to describe refraction, apply Snell's law, calculate refractive index and critical angle, and explain total internal reflection with real-world applications.
折射是指光从一种透明介质进入另一种不同密度的介质时发生的弯曲现象。这是理解透镜工作原理、吸管在水中看起来变弯以及光纤传输信息的核心原理。在 CCEA GCSE 物理大纲中，你需要能够描述折射、应用斯涅尔定律、计算折射率和临界角，并解释全内反射及其实际应用。
1. What is Refraction? | 什么是折射？
When a ray of light travels from one medium into another, its speed changes. If it enters the new medium at an angle (not along the normal), the change in speed causes the ray to change direction. This bending of light is called refraction.
当光线从一种介质进入另一种介质时，它的速度会发生变化。如果光线以某个角度（不沿法线方向）入射，速度的变化会导致光线改变方向。这种光的弯曲现象就称为折射。
The ray of light approaching the boundary is the incident ray, and the ray that has passed through is the refracted ray. The imaginary line perpendicular to the boundary at the point of incidence is called the normal.
射向界面的光线称为入射光线，穿过界面后的光线称为折射光线。在入射点处垂直于界面的假想线称为法线。
2. The Laws of Refraction | 折射定律
Refraction follows two main rules: 1) The incident ray, the refracted ray and the normal all lie in the same plane. 2) When light travels from a less dense to a more dense medium (e.g. air to glass), it bends towards the normal. Conversely, when light travels from a denser to a less dense medium (e.g. glass to air), it bends away from the normal.
折射遵循两条主要规则：1）入射光线、折射光线和法线在同一平面内。2）当光从光疏介质进入光密介质时（例如空气到玻璃），光线向法线弯曲。相反，当光从光密介质进入光疏介质时（例如玻璃到空气），光线会偏离法线弯曲。
ly along the normal (angle of incidence = 0°), it passes straight through without bending.
如果光线恰好沿法线方向入射（入射角 = 0°），它会直接穿过而不发生弯曲。
3. Refractive Index (n) and Snell's Law | 折射率与斯涅尔定律
The refractive index, n, of a medium is a measure of how much the speed of light is reduced inside that medium and how much the light bends. For light passing from a vacuum (or air) into the medium, the absolute refractive index is given by Snell's law:
折射率 n 是衡量光在该介质中速度减慢了多少以及光线弯曲程度的量。对于光从真空（或空气）进入介质的情况，绝对折射率由斯涅尔定律给出：
n = sin i / sin r
where i is the angle of incidence (in air) and r is the angle of refraction (in the medium).
其中 i 是入射角（空气中），r 是折射角（介质中）。
Snell' y two media, but at GCSE level using n = sin i / sin r for air to substance is sufficient.
斯涅尔定律也可以写成 n₁ sin θ₁ = n₂ sin θ₂ 用于任意两种介质，但在 GCSE 阶段，使用空气到物质的 n = sin i / sin r 就足够了。
air is approximately 1.00. Glass typically has n ≈ 1.5, water ≈ 1.33, and diamond ≈ 2.42.
空气的折射率约为 1.00。玻璃通常 n ≈ 1.5，水 ≈ 1.33，钻石 ≈ 2.42。
4. How to Measure the Refractive Index of Glass (Required Practical) | 如何测量玻璃的折射率（必做实验）
A standard CCEA practical uses a rectangular glass block, ray box, protractor and white paper. You trace the outline of the block, shine a light ray at an angle to the face, and mark the incident and emergent rays. You then measure i and r for different angles, calculate sin i and sin r, and plot a graph of sin i against sin r. The gradient of the straight line gives the refractive index n.
CCEA 标准实验使用矩形玻璃块、光线盒、量角器和白纸。描出玻璃块的轮廓，将光线以一定角度射向块面，标记入射光线和出射光线。然后测量不同角度下的 i 和 r，计算 sin i 和 sin r，并绘制 sin i 对 sin r 的图像。直线的斜率即为折射率 n。
te n = sin i / sin r for each pair and find the average, discarding any anomalous results.
也可以计算每对数据的 n = sin i / sin r，然后求平均值，舍弃异常结果。
5. Light Traveling from Denser to Rarer Medium | 光从光密介质到光疏介质
When light goes from a denser medium to a rarer one (e.g. glass to air), it speeds up and bends away from the normal. The angle of refraction r is larger than the angle of incidence i.
当光从光密介质进入光疏介质时（例如玻璃到空气），光速增大，光线偏离法线弯曲。折射角 r 大于入射角 i。
As the angle of incidence increases, the refracted ray gets closer to the boundary (r approaches 90°). At a particular angle of incidence, the refracted ray emerges along the boundary - this angle is the critical angle.
随着入射角增大，折射光线越来越贴近界面（r 趋向 90°）。在某一个特定入射角下，折射光线沿界面射出——这个角就是临界角。
6. Critical Angle and Total Internal Reflection | 临界角与全内反射
The critical angle, c, is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°.
临界角 c 是光在光密介质中的入射角，此时在光疏介质中的折射角为 90°。
If the angle of incidence is greater than the critical angle (i > c), all the light is reflected back into the denser medium. This is called total internal reflection (TIR). No refraction occurs.
如果入射角大于临界角（i > c），所有光线都会反射回光密介质中，这称为全内反射。不发生折射。
Two conditions must be met for TIR: the light must travel from a denser to a rarer medium, and the angle of incidence must exceed the critical angle.
发生全内反射必须满足两个条件：光必须从光密介质射向光疏介质，且入射角必须超过临界角。
7. Calculating Critical Angle: sin c = 1/n | 计算临界角
The critical angle is linked to refractive index by:
临界角与折射率的关系为：
sin c = 1 / n
d n is the refractive index of the denser medium (when the rarer medium is air, n ≈ 1.00).
其中 c 为临界角，n 为光密介质的折射率（当光疏介质为空气时，n ≈ 1.00）。
For example, for glass with n = 1.5, sin c = 1/1.5 = 0.667, so c ≈ 41.8°.
例如，折射率 n = 1.5 的玻璃，sin c = 1/1.5 = 0.667，因此 c ≈ 41.8°。
A higher refractive index means a smaller critical angle, making TIR easier to achieve.
折射率越高，临界角越小，全内反射越容易发生。
8. Applications of Total Internal Reflection | 全内反射的应用
Optical fibres use total internal reflection to transmit light signals over long distances. The fibre has a high-refractive-index core surrounded by a lower-refractive-index cladding. Light entering at one end strikes the core-cladding boundary at angles greater than the critical angle and is repeatedly totally internally reflected, travelling along the fibre with very little loss.
光纤利用全内反射远距离传输光信号。光纤有一个高折射率的纤芯，外面包着低折射率的包层。光线从一端进入，以大于临界角的角度射到纤芯与包层的界面，反复发生全内反射，沿着光纤传播，损耗极小。
Endoscopes use bundles of optical fibres to allow doctors to see inside the body. Prisms are used in binoculars and periscopes to reflect light by TIR because reflection by a prism is more efficient than mirrors and does not tarnish.
内窥镜使用光纤束让医生看到体内情况。棱镜在双筒望远镜和潜望镜中利用全内反射来反射光线，因为棱镜反射比平面镜效率更高，而且不会失去光泽。
9. Refraction and the Visible Spectrum (Dispersion) | 折射与可见光谱（色散）
White light is made up of a spectrum of colours. When white light enters a glass prism, each colour is refracted by a slightly different amount because the refractive index of glass varies with wavelength. Violet light slows down the most and is bent the most; red light is bent the least. This splitting of white light is called dispersion.
白光由一系列颜色组成。当白光进入玻璃棱镜时，每种颜色的光折射程度略有不同，因为玻璃的折射率随波长变化。紫光减速最多，弯曲最明显；红光弯曲最少。这种白光的分解称为色散。
The sequence of colours is: red, orange, yellow, green, blue, indigo, violet (ROYGBIV).
颜色顺序为：红、橙、黄、绿、蓝、靛、紫。
10. Everyday Effects of Refraction | 日常生活中的折射现象
A straw in a glass of water appears bent at the water surface because light from the submerged part travels from water to air, refracting away from the normal. Our brain interprets the light as having travelled in a straight line, so the straw seems to be at a shallower position.
吸管在水中看起来弯折，是因为水下部分的光线从水进入空气时，偏离法线折射。我们的大脑认为光线沿
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