Refraction of Light: IB WJEC Physics Key Concepts | IB WJEC 物理：光的折射 考点精讲

📚 Refraction of Light: IB WJEC Physics Key Concepts | IB WJEC 物理：光的折射 考点精讲

Refraction is a vital topic for both IB and WJEC physics, describing how waves change speed and direction when passing from one medium to another. Understanding Snell’s law, refractive index, total internal reflection, and practical applications like optical fibres and prisms will help you tackle exam questions confidently.

折射是 IB 和 WJEC 物理中至关重要的专题，描述波从一种介质进入另一种介质时速度和方向如何变化。理解斯涅尔定律、折射率、全内反射以及光纤和棱镜等实际应用，将帮助你自信应对考试题目。

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1. Introduction to Refraction | 折射简介

When a wave travels from one transparent medium to another, its speed changes due to the difference in optical density. If the wave hits the boundary at an angle other than 90°, this speed change causes the wave to change direction – a phenomenon called refraction.

当波从一种透明介质传播到另一种透明介质时，由于光密度的差异，其速度会改变。如果波以非90°角撞击界面，这种速度变化会导致波改变方向——这就是折射现象。

The frequency of the wave remains unchanged during refraction, but the wavelength adjusts according to v = fλ. Thus, a decrease in speed results in a shorter wavelength.

折射过程中波的频率保持不变，但波长会根据 v = fλ 进行调整。因此，速度减小会导致波长变短。

For light, refraction is responsible for many everyday effects, such as a straw appearing bent in a glass of water and the formation of rainbows.

对于光来说，折射是许多日常现象的原因，例如吸管在水中看起来弯曲以及彩虹的形成。

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2. Snell’s Law and Refractive Index | 斯涅尔定律与折射率

Snell’s law quantitatively describes refraction. It states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for a given pair of media, and is equal to the inverse ratio of the speeds or the ratio of the refractive indices.

斯涅尔定律定量地描述了折射。它表明，对于给定的一对介质，入射角的正弦与折射角的正弦之比是一个常数，等于速度的反比或折射率之比。

n₁ sin θ₁ = n₂ sin θ₂

where n₁ and n₂ are the absolute refractive indices of the first and second medium, θ₁ is the angle of incidence and θ₂ is the angle of refraction, both measured from the normal.

其中 n₁ 和 n₂ 分别是第一种介质和第二种介质的绝对折射率，θ₁ 是入射角，θ₂ 是折射角，均从法线量起。

The refractive index n of a medium is defined as the ratio of the speed of light in vacuum c to the speed of light in the medium v: n = c / v. This value is always greater than or equal to 1.

介质的折射率 n 定义为真空中的光速 c 与介质中的光速 v 之比：n = c / v。这个值总是大于或等于 1。

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3. Direction of Bending | 偏向法线还是远离法线

When light passes from a medium of lower refractive index (faster speed) to a medium of higher refractive index (slower speed), it bends towards the normal. For example, from air (n ≈ 1.00) into glass (n ≈ 1.50).

当光从折射率较低（速度较快）的介质进入折射率较高（速度较慢）的介质时，它会偏向法线。例如，从空气 (n ≈ 1.00) 进入玻璃 (n ≈ 1.50)。

Conversely, when moving from a higher-index medium to a lower-index medium, light bends away from the normal. This is observed when light exits glass into air.

相反，当从高折射率介质进入低折射率介质时，光线会远离法线弯曲。这在光从玻璃射入空气时可以观察到。

A quick way to remember: Light always takes the path that reduces travel time, as described by Fermat’s principle, resulting in the shortest optical path.

一个快速记忆法：根据费马原理，光总是选择减少旅行时间的路径，从而导致最短光程。

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4. Critical Angle and Total Internal Reflection | 临界角与全内反射

When light travels from a denser medium to a less dense medium (n₁ > n₂), there is a specific angle of incidence, called the critical angle θc, for which the angle of refraction is 90°. For angles of incidence greater than θc, all light is reflected back into the denser medium, a phenomenon known as total internal reflection (TIR).

当光从光密介质射向光疏介质 (n₁ > n₂) 时，存在一个特定的入射角，称为临界角 θc，此时折射角为 90°。若入射角大于 θc，则所有光线全被反射回光密介质，这种现象称为全内反射 (TIR)。

sin θc = n₂ / n₁ (where n₁ > n₂)

The critical angle depends only on the ratio of the refractive indices. Two conditions must be met for TIR: light must travel from a higher to lower refractive index, and the angle of incidence must exceed θc.

临界角仅取决于折射率之比。发生全内反射必须满足两个条件：光线必须从高折射率介质射向低折射率介质，且入射角必须大于临界角。

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5. Optical Fibres and TIR Applications | 光纤与全内反射应用

Optical fibres use total internal reflection to transmit light signals over long distances with minimal loss. A typical optical fibre consists of a core with a higher refractive index surrounded by cladding with a lower refractive index. Light entering the core at an angle greater than the critical angle is continuously reflected along the fibre.

光纤利用全内反射以极低的损耗长距离传输光信号。典型的光纤由折射率较高的纤芯和折射率较低的包层组成。以大于临界角射入纤芯的光线会沿光纤不断反射。

Other applications of TIR include prismatic periscopes, retroreflectors (like bicycle reflectors), and endoscopes used in medicine. TIR is more efficient than metallic reflection because almost no energy is absorbed.

其他 TIR 应用包括棱镜潜望镜、回射器（如自行车反射器）以及医学内窥镜。全内反射比金属反射更高效，因为几乎没有能量被吸收。

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6. Dispersion and the Prism | 色散与棱镜

Dispersion occurs because the refractive index of a material varies slightly with the wavelength (colour) of light. In glass, shorter wavelengths (blue light) travel slower and are refracted more than longer wavelengths (red light). When white light passes through a glass prism, it splits into a spectrum of colours – red, orange, yellow, green, blue, indigo, violet.

色散的产生是因为材料的折射率随光的波长（颜色）略有变化。在玻璃中，较短波长（蓝光）传播较慢，折射程度大于较长波长（红光）。当白光通过玻璃棱镜时，它被分解成红、橙、黄、绿、蓝、靛、紫的彩色光谱。

A prism demonstrates that white light is a mixture of different colours, and that each colour is bent by a different amount, with violet deviating the most and red the least. This is directly linked to the wavelength dependence of the refractive index.

棱镜证明白光是由不同颜色组成的混合物，每种颜色的偏折量不同，紫光偏折最大，红光最小。这与折射率对波长的依赖性直接相关。

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7. Refraction in Lenses | 透镜中的折射

Lenses use the principle of refraction at curved surfaces to converge or diverge light. A convex lens (thicker in the middle) refracts parallel rays to meet at a focal point, while a concave lens (thinner in the middle) spreads them apart.

透镜利用曲面上的折射原理来会聚或发散光线。凸透镜（中间厚）将平行光线折射后会聚于焦点，而凹透镜（

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