Refraction of Light for IGCSE Edexcel Physics | 光的折射考点精讲

📚 Refraction of Light for IGCSE Edexcel Physics | 光的折射考点精讲

Refraction is a core topic in the IGCSE Edexcel Physics specification. Understanding how light bends, Snell’s law, critical angle, and total internal reflection is essential for exam success. This guide breaks down every key concept with clear explanations, practical tips, and a worked example.

折射是IGCSE Edexcel物理课程的核心考点。理解光的弯曲、斯涅尔定律、临界角和全内反射对考试成功至关重要。本指南通过清晰的解释、实用技巧和例题精讲，逐一剖析每一个关键概念。

1. What is Refraction? | 什么是折射？

Refraction is the bending of light as it passes from one transparent medium to another due to a change in its speed. When light enters a denser medium (e.g., air to glass), it slows down and bends towards the normal. When it enters a rarer medium (e.g., glass to air), it speeds up and bends away from the normal.

折射是光在穿过不同透明介质时因传播速度改变而发生弯曲的现象。当光进入光密介质（如空气到玻璃）时，速度减慢并向法线偏折；进入光疏介质（如玻璃到空气）时，速度加快并偏离法线。

2. The Laws of Refraction | 折射定律

There are two laws of refraction. First, the incident ray, the refracted ray, and the normal all lie in the same plane. Second, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for a given pair of media, expressed as Snell’s law.

折射有两条定律。第一，入射光线、折射光线和法线位于同一平面内。第二，对于给定的两种介质，入射角的正弦与折射角的正弦之比是一个常数，即斯涅尔定律。

n₁ sin θ₁ = n₂ sin θ₂

3. Refractive Index | 折射率

The refractive index (n) of a medium is defined as the ratio of the speed of light in vacuum (c) to its speed in the medium (v): n = c / v. It indicates how much the medium slows light down. A higher refractive index means the medium is optically denser.

折射率 (n) 定义为光在真空中的速度 (c) 与它在介质中的速度 (v) 之比：n = c / v。它反映介质使光减速的程度。折射率越高，介质的光密度越大。

Medium	Refractive Index (n)
Air	1.00
Water	1.33
Glass (crown)	~1.50
Diamond	2.42

上表给出了常见介质的近似折射率，供快速参考。

4. Snell’s Law in Calculations | 斯涅尔定律的计算

Use n₁ sin θ₁ = n₂ sin θ₂, where θ₁ and θ₂ are the angles measured from the normal. This equation allows you to find an unknown angle or refractive index when a ray crosses a boundary. Always set your calculator to degree mode.

使用公式 n₁ sin θ₁ = n₂ sin θ₂，其中 θ₁ 和 θ₂ 是从法线量起的角度。该方程可用于求解光线穿越界面时的未知角度或折射率。务必确保计算器处于角度制模式。

When light enters along the normal (θ₁ = 0°), sin 0° = 0, so θ₂ = 0° – no bending occurs, but speed still changes.

当光沿法线入射时 (θ₁ = 0°)，sin 0° = 0，因此 θ₂ = 0°——不发生偏折，但速度仍然会改变。

5. Refraction from Denser to Rarer Medium | 光从光密到光疏介质

When light travels from a denser to a rarer medium (e.g., glass to air), the angle of refraction is always greater than the angle of incidence. As the incidence angle increases, the refracted ray approaches the boundary. At a certain incident angle, the refracted ray emerges exactly along the boundary – the angle of refraction is 90°.

当光从光密介质射向光疏介质（如玻璃到空气）时，折射角始终大于入射角。随着入射角的增大，折射光线越来越靠近界面。在某一特定入射角下，折射光线刚好沿界面射出——此时折射角为 90°。

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

The critical angle (θc) is the angle of incidence for which the angle of refraction is 90°. Applying Snell’s law: n₁ sin θc = n₂ sin 90°, so sin θc = n₂ / n₁, where n₁ > n₂. If the angle of incidence is greater than θc, total internal reflection occurs – all light is reflected back into the denser medium, obeying the law of reflection.

临界角 (θc) 是使折射角为 90° 的入射角。应用斯涅尔定律：n₁ sin θc = n₂ sin 90°，可得 sin θc = n₂ / n₁（其中 n₁ > n₂）。若入射角大于 θc，就会发生全内反射：所有光线遵循反射定律被全部反射回光密介质。

sin θc = n₂ / n₁

Two conditions must be satisfied: light must travel from a denser to a rarer medium, and the angle of incidence must exceed the critical angle.

必须满足两个条件：光从光密介质射向光疏介质，且入射角大于临界角。

7. Optical Fibres and Applications | 光纤及其应用

Optical fibres rely on total internal reflection to transmit light pulses over long distances with minimal signal loss. A fibre consists of a high-refractive-index core surrounded by a lower-index cladding. Light that enters the core at an angle greater than the critical angle is continuously reflected along the fibre. Applications include high-speed internet, telephone cables, and medical endoscopes that allow doctors to see inside the body.

光纤依赖全内反射，以极低的信号损耗长距离传输光脉冲。光纤由高折射率纤芯和低折射率包层构成。以大于临界角的角度进入纤芯的光线会沿光纤不断反射。其应用包括高速互联网、电话电缆，以及让医生观察人体内部的医用内窥镜。

8. Investigating Refraction: Experiment | 探究折射实验

A standard practical uses a rectangular glass block. Shine a narrow ray of light at an angle through the block, trace the path of the incident and emergent rays, and mark the points. Remove the block, draw the rays, and measure the angles of incidence (i) and refraction (r) with a protractor. Repeat for different angles, then plot a graph of sin i against sin r; the gradient equals the refractive index of the glass, verifying Snell’s law.

一个标准实验使用矩形玻璃砖。让一束细光线以一定角度射入玻璃砖，追踪入射光线和出射光线的路径并标点。移开玻璃砖，画出光线，用量角器测量入射角 (i) 和折射角 (r)。改变入射角重复测量，然后绘制 sin i – sin r 图线；图像斜率即为玻璃的折射率，从而验证斯涅尔定律。

The emergent ray is parallel to the incident ray but laterally displaced. This occurs because the two parallel faces reverse the bending effect.

出射光线与入射光线平行但发生了横向位移。这是因为两个平行面将偏折效果逆转了。

9. Common Misconceptions and Exam Tips | 常见误区与考试技巧

Angles are always measured between the ray and the normal, not the boundary. A common error is taking the angle from the surface. Remember that frequency of light stays constant when entering a new medium; wavelength and speed change, causing the bending.

角度总是光线与法线之间的夹角，而不是与界面的夹角。常见错误是误用量角器从界面测量。记住，进入新介质时光的频率保持不变；波长和速度发生变化，从而引起折射。

If a ray strikes the boundary at exactly the critical angle, the refracted ray runs along the boundary (angle 90°). For angles beyond the critical angle, there is no refracted ray – only reflection. In ray diagrams, always draw arrows on the rays, label the normal, and write angles clearly.

若光线以恰好临界角射入界面，折射光线将沿界面传播（折射角 90°）。超过临界角时，就没有折射光线了——只有反射。在光路图中，务必在光线上画箭头，标出法线，并清晰标明角度。

10. Worked Example | 例题精讲

A ray of light travels from water (n = 1.33) into air (n = 1.00). If the angle of incidence in water is 30°, calculate the angle of refraction in air.

一束光从水 (n = 1.33) 射入空气 (n = 1.00)。水中的入射角为 30°，试求空气中的折射角。

Solution: Use n₁ sin θ₁ = n₂ sin θ₂. Let medium 1 be water (n₁ = 1.33, θ₁ = 30°) and medium 2 be air (n₂ = 1.00).

解：运用 n₁ sin θ₁ = n₂ sin θ₂。设水为介质1 (n₁ = 1.33, θ₁ = 30°)，空气为介质2 (n₂ = 1.00)。

1.33 × sin 30° = 1.00 × sin θ₂
1.33 × 0.5 = sin θ₂
sin θ₂ = 0.665
θ₂ = sin⁻¹(0.665) ≈ 41.7°

Therefore, the refracted ray in air makes an angle of about 41.7° with the normal. The ray bends away from the normal, as expected when light enters a rarer medium.

因此，空气中的折射光线与法线夹角约为 41.7°。正如光进入光疏介质时所料，光线偏离了法线。

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Refraction of Light for IGCSE Edexcel Physics | 光的折射 考点精讲