IGCSE OCR Physics: Refraction of Light | IGCSE OCR 物理：光的折射考点精讲

📚 IGCSE OCR Physics: Refraction of Light | IGCSE OCR 物理：光的折射考点精讲

Refraction is a fundamental wave property that explains how light changes direction when it travels from one medium to another, such as from air to glass. Understanding refraction is crucial for the IGCSE OCR Physics exam, as it links wave theory with practical applications like lenses, optical fibres, and even the formation of rainbows. This article covers all the essential points, from Snell’s law and refractive index to total internal reflection.

折射是波的基本性质，解释了光从一种介质进入另一种介质（如从空气到玻璃）时方向如何改变。理解折射对于 IGCSE OCR 物理考试至关重要，因为它将波的理论与透镜、光纤甚至彩虹形成等实际应用联系起来。本文将涵盖所有重要考点，从斯涅尔定律和折射率到全内反射。

1. What is Refraction? | 什么是折射？

When a wave crosses a boundary between two media at an angle, its speed changes. Because one side of the wavefront enters the new medium before the other, the wave bends. This bending is called refraction.

当波以一定角度穿过两种介质之间的界面时，其速度会改变。由于波前的一侧先于另一侧进入新介质，波会发生弯曲。这种弯曲现象就叫做折射。

For light, refraction only occurs at an angle; if the light ray hits the boundary along the normal (perpendicular), it passes straight through without bending, though its speed still changes.

对于光，折射仅在斜射时发生；如果光线沿着法线（垂直）方向射向界面，它将直线穿过而不发生偏折，尽管速度仍然发生了改变。

Refraction explains many everyday observations, such as a straw appearing bent in a glass of water or a swimming pool looking shallower than it really is.

折射解释了许多日常观察现象，例如水杯中的吸管看起来是弯的，或者游泳池看起来比实际浅。

2. The Laws of Refraction | 折射定律

There are two laws of refraction:

折射有两条定律：

The incident ray, the refracted ray and the normal all lie in the same plane.

入射光线、折射光线和法线位于同一平面内。
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. This is known as Snell’s law.

对于给定的一对介质，入射角的正弦与折射角的正弦之比为一个常数。这就是斯涅尔定律。

Mathematically, this is expressed as:

数学表达为：

n = sin i / sin r

where i is the angle of incidence and r is the angle of refraction, both measured from the normal. The constant n is the refractive index of the second medium with respect to the first.

其中 i 是入射角，r 是折射角，均从法线量起。常数 n 是第二种介质相对于第一种介质的折射率。

3. Refractive Index | 折射率

The refractive index n of a material measures how much light slows down in that material. It is defined as the ratio of the speed of light in a vacuum c to the speed of light in the material v:

材料的折射率 n 衡量了光在该材料中减慢的程度。它被定义为真空中光速 c 与材料中光速 v 之比：

n = c / v

Since v is always less than c, n is always greater than 1 for any material (except vacuum where n=1). A higher refractive index means light travels more slowly in that medium and bends more towards the normal when entering from air.

由于 v 总是小于 c，对于任何材料，n 总是大于 1（真空除外，n=1）。折射率越高，意味着光在这种介质中传播越慢，从空气射入时向法线偏折的程度越大。

Also, for any pair of media, n₁ sin θ₁ = n₂ sin θ₂, where n₁ and n₂ are the refractive indices of the first and second media.

此外，对于任意两种介质，有 n₁ sin θ₁ = n₂ sin θ₂，其中 n₁ 和 n₂ 分别是第一种和第二种介质的折射率。

In the IGCSE exam, you are often given the refractive index of glass as 1.5 or water as 1.33.

在 IGCSE 考试中，通常会给出玻璃的折射率 1.5 或水的折射率 1.33。

4. Snell’s Law in Action | 斯涅尔定律应用

When solving problems, you will often use n = sin i / sin r, where i and r are the angles measured from the normal. For a ray going from air (n≈1) into glass (n=1.5), if the angle of incidence is 30°, you can find the angle of refraction.

解题时，你通常会使用 n = sin i / sin r，其中 i 和 r 是从法线量起的角度。对于从空气（n≈1）进入玻璃（n=1.5）的光线，若入射角为 30°，你可以求出折射角。

Example calculation: sin r = sin i / n = sin 30° / 1.5 = 0.5 / 1.5 ≈ 0.333, so r ≈ 19.5°.

计算示例：sin r = sin i / n = sin 30° / 1.5 = 0.5 / 1.5 ≈ 0.333，所以 r ≈ 19.5°。

Remember that if light enters a denser medium (higher n), it bends towards the normal; if it enters a less dense medium, it bends away from the normal.

记住，光进入光密介质（n 较大）时向法线偏折；进入光疏介质时偏离法线。

Always ensure your calculator is in degree mode, and round final answers to an appropriate number of significant figures as per the question.

始终保持计算器处于角度模式，并根据题目要求将最终答案四舍五入到适当有效数字。

5. Light Entering Different Media | 光进入不同介质

The behavior of light at a boundary depends on the optical densities. A medium with a higher refractive index is optically denser. When light goes from a less dense to a denser medium, it slows down and bends towards the normal. When it goes from denser to less dense, it speeds up and bends away from the normal.

光在界面处的行为取决于光密介质。折射率较高的介质是光密介质。当光从光疏介质射向光密介质时，速度减慢并靠近法线折射；当从光密介质射向光疏介质时，速度加快并远离法线折射。

A useful table summarising the changes:

以下表格总结了相关变化：

Direction	Speed change	Wavelength change	Frequency change	Bending
Air to glass (less to more dense)	decreases	decreases	constant	towards normal
Glass to air (more to less dense)	increases	increases	constant	away from normal

Note that the frequency of the light remains the same because it depends only on the source.

注意光的频率保持不变，因为它只取决于光源。

6. Refraction and Wave Properties | 折射与波的性质

Since v = f λ (wave speed = frequency × wavelength), and frequency f is constant when light enters a new medium, any change in speed v must be accompanied by a proportional change in wavelength λ.

由于 v = f λ（波速 = 频率 × 波长），且频率 f 在光进入新介质时不变，因此速度 v 的任何变化必然伴随着波长 λ 的相应变化。

In a denser medium, speed decreases, so wavelength decreases, causing the wavefronts to bunch up. This can be visualised using a ripple tank simulation where water waves slow down in shallower water and bend.

在光密介质中，速度减小，因此波长减小，导致波前聚集。这可以通过波纹槽模拟来直观感受：水波在较浅的水中变慢并发生弯曲。

Understanding this helps explain why the colour of light does not change during refraction — colour is determined by frequency, which remains constant.

理解这一点有助于解释为什么光的颜色在折射过程中不会改变——颜色由频率决定，而频率保持不变。

7. Critical Angle | 临界角

When light travels from a denser medium to a less dense medium, there is a certain angle of incidence for which the angle of refraction becomes 90°. This incidence angle is called the critical angle (c).

当光从光密介质射向光疏介质时，存在某个入射角，使得折射角变为 90°。这个入射角称为临界角（c）。Published by TutorHao | IGCSE Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

IGCSE OCR Physics: Refraction of Light | IGCSE OCR 物理：光的折射 考点精讲