Sound and Light Waves: A KS3 Cambridge Physics Guide — 声与光：剑桥初中物理波动基础指南

📚 Sound and Light Waves: A KS3 Cambridge Physics Guide | 声与光：剑桥初中物理波动基础完全指南

波是物理学中最重要也最神奇的概念之一。你说话时，声波从你的嘴巴传向朋友的耳朵；你照镜子时，光波从你的脸反射回你的眼睛。声音和光都是波，但它们的行为却非常不同。这篇文章将带你系统地学习剑桥初中物理（Cambridge Lower Secondary Physics）中关于声与光的全部核心知识：什么是波、横波与纵波的区别、声音如何产生与传播、音调与响度由什么决定、光如何反射与折射，以及为什么天空是蓝色的。学完这篇文章，你不仅能应付考试，还能用物理的眼光重新看待身边的世界。

Waves are one of the most important and fascinating ideas in physics. When you speak, sound waves travel from your mouth to your friend’s ear. When you look in a mirror, light waves bounce off your face and return to your eyes. Sound and light are both waves, yet they behave in very different ways. This article will take you systematically through the core knowledge of sound and light in Cambridge Lower Secondary Physics: what a wave is, the difference between transverse and longitudinal waves, how sound is produced and travels, what determines pitch and loudness, how light reflects and refracts, and why the sky is blue. By the end, you will not only be ready for your exams, but you will also see the world around you through the eyes of a physicist.

1. 什么是波：振动的传播 | What Is a Wave? How Vibrations Travel

波的本质是能量的传递，而不是物质的移动。想象一下：你把一块小石子扔进平静的池塘，水面会出现一圈圈向外扩散的波纹。水面上的树叶并不会跟着波纹漂到池塘中央，它只是在原地上下晃动。真正向外传播的是能量，水分子本身只是在平衡位置附近来回振动。这就是波的核心定义：波是一种通过振动把能量从一处传递到另一处的过程，而传播波动的物质本身并没有整体移动。

A wave is a way of transferring energy, not a movement of matter. Imagine dropping a small stone into a calm pond: circular ripples spread outward across the surface. A leaf floating on the water does not drift to the centre of the pond; it simply bobs up and down in place. What actually travels outward is energy. The water molecules themselves only vibrate around their equilibrium positions. This is the core definition of a wave: a wave is a process that transfers energy from one place to another through vibrations, while the material carrying the wave does not move as a whole.

描述波有三个关键量：波长、频率和振幅。波长（wavelength）是一个完整波的长度，通常用希腊字母 λ 表示；频率（frequency）是每秒钟完成的完整波数，单位是赫兹（Hz）；振幅（amplitude）是振动点离开平衡位置的最大距离，它决定了波携带的能量大小。振幅越大，波的能量越强，听起来越响，看起来越亮。记住这三兄弟，后面所有的内容都离不开它们。

There are three key quantities used to describe a wave: wavelength, frequency and amplitude. Wavelength is the length of one complete wave, usually represented by the Greek letter lambda. Frequency is the number of complete waves produced each second, measured in hertz (Hz). Amplitude is the maximum distance a vibrating point moves from its equilibrium position, and it determines how much energy the wave carries. The larger the amplitude, the more energy the wave carries, the louder it sounds and the brighter it looks. Remember these three partners, because everything in the rest of this article depends on them.

Quantity 物理量	What It Means 含义	Unit 单位
Wavelength 波长	Length of one complete wave 一个完整波的长度	metre (m) 米
Frequency 频率	Number of complete waves per second 每秒完成的波数	hertz (Hz) 赫兹
Amplitude 振幅	Maximum distance from equilibrium 离开平衡位置的最大距离	metre (m) 米

2. 横波与纵波：两种主要的波动方式 | Transverse and Longitudinal Waves: Two Ways to Wave

根据振动方向与传播方向的关系，波可以分为两大类：横波和纵波。在横波（transverse wave）中，粒子的振动方向与波的传播方向垂直。想象你把一根绳子的一端上下抖动，绳子上的波沿着绳子水平传播，而每个小段绳子却在上下振动，方向正好成直角。水波、光波和所有电磁波都是横波。

Waves can be divided into two main families according to how the vibration direction relates to the direction of travel: transverse waves and longitudinal waves. In a transverse wave, the particles vibrate at right angles (perpendicular) to the direction in which the wave travels. Imagine shaking one end of a rope up and down: the wave travels horizontally along the rope, but every small section of the rope vibrates vertically, at exactly right angles to the motion. Water waves, light waves and all electromagnetic waves are transverse waves.

在纵波（longitudinal wave）中，粒子的振动方向与波的传播方向平行，即沿着同一条直线前后振动。纵波由一列压缩区（compressions）和稀疏区（rarefactions）组成：压缩区是粒子挤在一起的区域，稀疏区是粒子被拉开的区域。声音在空气和水中传播时就是纵波。你可以用弹簧（slinky）来演示纵波：快速推一下弹簧的一端，会看到一个密集的线圈沿着弹簧传播。

In a longitudinal wave, the particles vibrate parallel to the direction of travel, back and forth along the same straight line. A longitudinal wave consists of a series of compressions and rarefactions: compressions are regions where particles are squeezed together, and rarefactions are regions where particles are spread apart. Sound travels through air and water as a longitudinal wave. You can demonstrate this with a slinky spring: give one end a quick push and you will see a bunch of compressed coils travel along the spring.

Feature 特征	Transverse Wave 横波	Longitudinal Wave 纵波
Vibration direction 振动方向	Perpendicular to travel 与传播方向垂直	Parallel to travel 与传播方向平行
Structure 结构	Crests and troughs 波峰与波谷	Compressions and rarefactions 压缩区与稀疏区
Examples 例子	Light, water waves 光、水波	Sound waves 声波

3. 声音的产生：振动如何变成我们听到的声音 | How Sound Is Made: From Vibrations to What We Hear

声音是由振动产生的。当你拨动吉他弦时，弦在快速振动；当你说话时，声带在振动；当你敲鼓时，鼓面在振动。所有这些振动都会挤压周围的空气分子，形成一列压缩区和稀疏区，这就是声波。声波传播到你的耳朵，推动耳膜振动，大脑再把这种振动解读为声音。没有振动，就没有声音，这是声音产生的第一条铁律。

Sound is produced by vibrations. When you pluck a guitar string, the string vibrates rapidly. When you speak, your vocal cords vibrate. When you hit a drum, the drum skin vibrates. All these vibrations squeeze the air molecules around them, creating a train of compressions and rarefactions, and that is a sound wave. When the sound wave reaches your ear, it pushes your eardrum back and forth, and your brain interprets these vibrations as sound. No vibration, no sound: this is the first iron rule of sound production.

声音的传播需要介质。介质（medium）是波赖以传播的物质，可以是固体、液体或气体。在真空中没有空气分子可以振动，所以声音无法在真空中传播。这正是宇航员在太空中不能直接交谈的原因，他们必须通过无线电设备交流。你可以做一个著名的实验来验证这一点：把一个正在响铃的闹钟放进密封的玻璃罩里，用真空泵抽走罩内的空气，铃声会越来越小，最后完全听不见。

Sound needs a medium to travel through. A medium is the material that carries a wave, and it can be a solid, a liquid or a gas. In a vacuum there are no air molecules to vibrate, so sound cannot travel through a vacuum. This is exactly why astronauts cannot talk to each other directly in space; they must communicate using radio equipment. You can verify this with a famous experiment: place a ringing alarm clock inside a sealed glass jar, pump the air out with a vacuum pump, and the ringing becomes quieter and quieter until it disappears completely.

声音在固体中传播最快，在气体中最慢，因为固体中的粒子排列紧密，振动更容易传递给相邻粒子。这就是为什么你能在铁轨上提前听到远处火车开来的声音：把耳朵贴在铁轨上，声音比通过空气传来得更早。水的密度介于固体和气体之间，所以声音在水中比在空气中传播得快，这也是鲸鱼能在海洋中远距离交流的原因。

Sound travels fastest in solids and slowest in gases, because particles in solids are packed closely together and vibrations pass more easily to neighbouring particles. This is why you can hear an approaching train earlier by putting your ear to the railway track: the sound arrives through the steel rail before it arrives through the air. Water sits between solids and gases in density, so sound travels faster in water than in air. This is also why whales can communicate over huge distances in the ocean.

4. 音调与响度：频率和振幅的作用 | Pitch and Loudness: The Roles of Frequency and Amplitude

为什么女生的声音通常比男生的高？为什么用力敲鼓会比轻轻敲鼓更响？答案就在频率和振幅这两个量里。音调（pitch）由频率决定：频率越高，音调越高。女生的声带比男生的短而紧，振动得更快，所以发出的声音频率更高，音调也就更高。蚊子飞行时翅膀每秒振动数百次，发出尖锐的嗡嗡声，而大提琴的低音来自每秒只振动几十次的琴弦。

Why are women’s voices usually higher than men’s? Why does hitting a drum hard make it louder than tapping it gently? The answers lie in frequency and amplitude. Pitch is determined by frequency: the higher the frequency, the higher the pitch. Women’s vocal cords are shorter and tighter than men’s, so they vibrate faster, producing a higher frequency and therefore a higher pitch. A mosquito’s wings vibrate hundreds of times per second, producing the sharp buzzing sound, while the low notes of a cello come from strings vibrating only tens of times per second.

响度（loudness）由振幅决定：振幅越大，声音越响。用力敲鼓时，鼓面振动得更剧烈，离开平衡位置更远，振幅更大，推动空气的力量更强，传到耳朵里的能量更多，所以我们听到的声音更响。轻声说话时声带振动幅度小，声音就轻。记住一个简单的关系：音调高不高看频率，声音响不响看振幅，两者互不影响。

Loudness is determined by amplitude: the larger the amplitude, the louder the sound. When you hit a drum hard, the skin vibrates more violently, moving further from its equilibrium position, so the amplitude is larger. The drum pushes the air with more force, more energy reaches your ear, and the sound is louder. When you whisper, your vocal cords vibrate with small amplitude, so the sound is soft. Remember this simple relationship: pitch depends on frequency, loudness depends on amplitude, and the two do not affect each other.

人耳能听到的声音频率范围大约是 20 Hz 到 20000 Hz。低于 20 Hz 的声波称为次声波（infrasound），高于 20000 Hz 的声波称为超声波（ultrasound）。人耳听不到超声波，但蝙蝠和海豚可以用超声波导航和捕食，医院也用超声波来检查婴儿在妈妈肚子里的情况。不同动物的听觉范围差异很大：狗能听到比人更高的声音，所以狗哨发出的高频声波人听不见，狗却能听见。

The range of frequencies audible to the human ear is roughly 20 Hz to 20000 Hz. Sound waves below 20 Hz are called infrasound, and waves above 20000 Hz are called ultrasound. Humans cannot hear ultrasound, but bats and dolphins use it to navigate and hunt, and hospitals use ultrasound scanning to check on babies inside their mothers. Different animals have very different hearing ranges: dogs can hear higher sounds than humans, which is why a dog whistle produces high-frequency sound that people cannot hear but dogs can.

5. 声速：声音在不同介质中传播的快慢 | The Speed of Sound: How Fast Sound Travels

在 20 摄氏度的空气中，声音的传播速度约为每秒 340 米。这个速度看起来很快，但和光速相比就慢得多了。闪电和雷声就是最好的例子：闪电的光几乎瞬间到达你的眼睛，而雷声需要几秒钟才传到你的耳朵。如果你数一下闪电和雷声之间的秒数，每三秒大约对应一千米的距离。这就是最简单的测距方法：距离（米）约等于秒数乘以 340。

In air at 20 degrees Celsius, sound travels at about 340 metres per second. That sounds fast, but it is extremely slow compared with the speed of light. Lightning and thunder are the perfect example: the flash of light reaches your eyes almost instantly, but the thunder takes several seconds to reach your ears. If you count the seconds between the flash and the thunder, every three seconds corresponds to roughly one kilometre of distance. This is the simplest way to measure distance: distance in metres is approximately equal to the number of seconds multiplied by 340.

Medium 介质	Speed of Sound 声速 (m/s)
Air (20°C) 空气	About 340 约 340
Water 水	About 1500 约 1500
Steel 钢	About 5000 约 5000

声速还会受到温度的影响：温度越高，空气中的分子运动越快，声音传播得越快。在寒冷的日子里，声速略低于 340 米每秒；在炎热的日子里，声速略高于 340 米每秒。考试中经常出现这样的计算题：一个人站在山谷中大喊一声，2 秒后听到回声，问山谷的峭壁离他多远。解题的关键是声音走了一个来回，所以距离等于声速乘以时间再除以二。

The speed of sound is also affected by temperature: the higher the temperature, the faster the air molecules move and the faster sound travels. On cold days the speed is slightly below 340 metres per second; on hot days it is slightly above. Exams often include a calculation like this: a person standing in a valley shouts once and hears the echo 2 seconds later. How far away is the cliff? The key is that the sound has travelled there and back, so the distance equals the speed of sound multiplied by the time, then divided by two.

6. 回声与超声：声音的反射及其应用 | Echoes and Ultrasound: Reflections of Sound in Action

回声（echo）是声音被坚硬表面反射回来的现象。当你对着远处的悬崖或大楼喊话时，声波传播到墙面后被反弹回来，你就能听到自己声音的回音。要听到清晰的回声，反射面必须离你足够远，一般至少 17 米，这样回声和原声之间的时间间隔超过 0.1 秒，人耳才能把它们区分开。如果反射面太近，回声会和原声混在一起，反而让声音听起来更响亮，这就是音乐厅和剧院设计墙壁形状的原理。

An echo is the reflection of sound from a hard surface. When you shout towards a distant cliff or building, the sound waves bounce off the wall and return, and you hear your own voice coming back. To hear a clear echo, the reflecting surface must be far enough away, generally at least 17 metres, so that the gap between the original sound and the echo is more than 0.1 seconds and the human ear can tell them apart. If the reflecting surface is too close, the echo blends with the original sound and simply makes it seem louder. This is the principle behind the curved wall designs of concert halls and theatres.

声呐（sonar）是回声原理最重要的应用之一。船上的声呐设备向海底发射超声波，声波碰到海底后反射回来，设备通过测量声波往返的时间就能计算出海水的深度。声呐还可以用来探测鱼群、绘制海底地图、帮助潜水艇导航。医院里的超声波扫描（B 超）原理相同：不同组织反射超声波的程度不同，电脑根据反射信号画出人体内部的图像，医生就能看到胎儿的发育情况而无需任何手术。

Sonar is one of the most important applications of the echo principle. A ship’s sonar system sends ultrasound towards the seabed; the waves reflect back when they hit the bottom, and the equipment calculates the depth of the water by measuring the time the waves take to travel there and back. Sonar is also used to detect schools of fish, map the ocean floor and help submarines navigate. Hospital ultrasound scanning works on the same principle: different tissues reflect ultrasound to different degrees, and a computer builds an image of the inside of the body from the reflected signals, allowing doctors to see how a baby is developing without any surgery.

7. 光的本质：一种传播极快的横波 | Light: A Very Fast Transverse Wave

光是一种横波，属于电磁波家族。和声波不同，光不需要介质，它在真空中传播得最快，速度约为每秒 30 万千米（3 乘以 10 的 8 次方米每秒）。光从太阳出发，大约只需要 8 分 20 秒就能到达地球，而这 1.5 亿千米的距离，声音要花 14 年才能走完。正是因为它不需要介质，太阳光才能穿过真空的太空照亮地球。

Light is a transverse wave and belongs to the electromagnetic wave family. Unlike sound, light does not need a medium: it travels fastest in a vacuum, at about 300000 kilometres per second (3 x 10^8 metres per second). Light from the Sun takes only about 8 minutes 20 seconds to reach the Earth, while sound would need about 14 years to cover the same 150 million kilometres. Because light needs no medium, sunlight can cross the vacuum of space to light up the Earth.

我们看到的太阳光是白光，但它其实是由多种颜色的光混合而成的。通过三棱镜，白光可以被分解成红、橙、黄、绿、蓝、靛、紫七种颜色，这个彩色光带叫作光谱（spectrum）。不同颜色的光波长不同：红光的波长最长，紫光的波长最短。彩虹就是大自然的三棱镜：雨滴把阳光折射并反射，把白光分解成七彩光带。

The sunlight we see is white light, but it is actually a mixture of many colours. When white light passes through a prism, it splits into red, orange, yellow, green, blue, indigo and violet, and this coloured band is called the spectrum. Different colours have different wavelengths: red light has the longest wavelength and violet light has the shortest. A rainbow is nature’s prism: raindrops refract and reflect sunlight, splitting white light into a band of seven colours.

8. 光的反射：平面镜中的世界 | Reflection of Light: Seeing Yourself in a Plane Mirror

为什么你能在镜子里看到自己？因为光在光滑的表面发生了反射。反射遵循两条定律：第一，入射角等于反射角；第二，入射光线、反射光线和法线都在同一平面内。这里的法线（normal）是一条假想的、垂直于反射面的线，入射角是入射光线与法线的夹角，反射角是反射光线与法线的夹角。注意，角度都是相对于法线测量的，而不是相对于镜面。

Why can you see yourself in a mirror? Because light reflects from a smooth surface. Reflection follows two laws: first, the angle of incidence equals the angle of reflection; second, the incident ray, the reflected ray and the normal all lie in the same plane. The normal is an imaginary line drawn perpendicular to the reflecting surface. The angle of incidence is the angle between the incident ray and the normal, and the angle of reflection is the angle between the reflected ray and the normal. Note that angles are always measured relative to the normal, not relative to the mirror surface.

平面镜中的像有三个特点：像与物体大小相同、像到镜面的距离等于物体到镜面的距离、像是左右颠倒的虚像。虚像（virtual image）的意思是光线并没有真正从那个位置发出，而是光线的反向延长线汇聚在那里，所以你在镜子里摸不到那个”像”。当你向镜子走近 1 米时，你与像之间的距离会缩短 2 米，因为你和你的像都在向镜面靠近。

The image in a plane mirror has three characteristics: the image is the same size as the object, the image distance equals the object distance, and the image is laterally inverted (left and right are swapped) and virtual. A virtual image means the light rays do not actually come from that position; they only appear to come from there because their backward extensions meet at that point. That is why you cannot touch the person in the mirror. When you walk one metre towards a mirror, the distance between you and your image shrinks by two metres, because both you and your image are moving towards the mirror.

镜面反射和漫反射也很重要。光滑的表面（如镜面）发生镜面反射，平行光被规则地反射到同一方向，所以你能看到清晰的像。粗糙的表面（如白纸、墙壁）发生漫反射，平行光被反射到各个方向，所以从任何角度都能看到被照亮的物体。我们能看到不发光的物体，正是因为它们把光漫反射到我们的眼睛里。

Specular reflection and diffuse reflection are also important. Smooth surfaces such as mirrors produce specular reflection: parallel rays are reflected regularly in the same direction, so you see a clear image. Rough surfaces such as white paper and walls produce diffuse reflection: parallel rays are scattered in all directions, so you can see an illuminated object from any angle. We can see objects that do not emit light precisely because they diffuse light into our eyes.

9. 光的折射：光线为什么会弯折 | Refraction: Why Light Bends

把一根笔直的吸管斜着插入一杯水中，你会发现吸管看起来在液面处折断了。这不是吸管真的断了，而是光发生了折射（refraction）。当光从一种透明介质进入另一种透明介质时，它的传播速度会改变，方向也随之偏折。光从空气进入水中时速度变慢，向法线方向弯折；光从水中进入空气时速度变快，偏离法线方向弯折。

Put a straight straw diagonally into a glass of water and it appears to bend at the surface. The straw is not really broken; the light has undergone refraction. When light passes from one transparent medium into another, its speed changes and its direction bends. When light travels from air into water it slows down and bends towards the normal; when it travels from water into air it speeds up and bends away from the normal.

折射的规律可以总结为：光从光疏介质进入光密介质时（如空气到水、空气到玻璃），折射角小于入射角，光线向法线靠拢；反过来，光从光密介质进入光疏介质时，折射角大于入射角，光线远离法线。如果入射角为 0 度，即光垂直射向界面，光不改变方向，直接穿过去。折射现象在生活中无处不在：游泳池看起来比实际浅，因为池底反射的光经过水面折射后进入眼睛；透过装满水的玻璃杯看铅笔，铅笔会显得又粗又歪。

The rule of refraction can be summarised as follows: when light travels from a less dense medium into a denser medium (such as air to water, or air to glass), the angle of refraction is smaller than the angle of incidence and the ray bends towards the normal. Conversely, when light travels from a denser medium into a less dense one, the angle of refraction is larger and the ray bends away from the normal. If the angle of incidence is 0 degrees, meaning the light strikes the boundary at right angles, the light passes straight through without changing direction. Refraction is everywhere in daily life: a swimming pool looks shallower than it really is because light from the bottom bends as it leaves the water and enters your eye; and a pencil viewed through a full glass of water looks thick and bent.

10. 光与颜色：彩虹背后的可见光谱 | Light and Colour: The Visible Spectrum Behind Rainbows

为什么苹果是红色的，树叶是绿色的？因为物体反射什么颜色的光，我们就看到什么颜色。白苹果皮？不，是白光照射苹果时，苹果皮吸收了除红色以外的所有颜色，只把红光反射进你的眼睛，所以苹果看起来是红的。绿叶吸收除绿色以外的光，反射绿光。黑色物体吸收所有颜色的光，不反射任何光；白色物体反射所有颜色的光，不吸收任何光。

Why is an apple red and a leaf green? Because we see the colour of light that an object reflects. When white light shines on an apple, the skin absorbs every colour except red and reflects only the red light into your eyes, so the apple looks red. A green leaf absorbs all colours except green and reflects the green light. Black objects absorb every colour and reflect nothing, while white objects reflect every colour and absorb nothing.

这也可以解释为什么在暗房里物体的颜色会改变。如果只用红光照射一个绿色的物体，绿色物体上没有绿光可以反射，它吸收红光后看起来几乎是黑色的。同样，在蓝色灯光下，红色的衣服会显得发黑。三原色（红、绿、蓝）可以通过不同比例混合出几乎所有颜色：红光加绿光得到黄光，绿光加蓝光得到青光，红光加蓝光得到品红光，三种光等量混合则得到白光。这就是电视和手机屏幕的成像原理，每个像素都由红绿蓝三个小灯组成。

This also explains why the colour of objects changes in a dark room. If you shine only red light on a green object, there is no green light for the object to reflect; it absorbs the red light and looks almost black. Similarly, a red shirt looks dark under blue light. The three primary colours of light (red, green and blue) can be mixed in different proportions to make almost any colour: red plus green makes yellow, green plus blue makes cyan, red plus blue makes magenta, and equal amounts of all three make white. This is how television and phone screens work: every pixel is made of three tiny lights, one red, one green and one blue.

11. 声与光对比：一张表看懂两种波 | Sound vs Light: One Table That Tells You Everything

声和光是考试中最常被放在一起比较的两种波，把它们的区别整理成一张表，复习效率会大大提高。下面这张表覆盖了考试最喜欢考的所有对比点，建议抄写进你的笔记本，考试前看一遍。

Sound and light are the two types of wave most often compared in exams. Turning their differences into a table makes revision much more efficient. The table below covers every comparison point that exams love to test, and we suggest copying it into your notebook and reading it once before each exam.

Feature 特征	Sound 声音	Light 光
Type of wave 波的种类	Longitudinal 纵波	Transverse 横波
Needs a medium? 需要介质吗	Yes, cannot travel in vacuum 需要，真空中不能传播	No, travels in vacuum 不需要，真空中传播最快
Speed in air 空气中的速度	About 340 m/s 约 340 米每秒	300000 km/s 30 万千米每秒
Speed comparison 速度比较	Fastest in solids 固体中最快	Fastest in vacuum 真空中最快
Audible range 可听范围	20 Hz to 20000 Hz 20 赫兹到 2 万赫兹	No range, all visible colours 无范围限制

12. 考点总结与练习：学以致用 | Exam Points and Practice Questions

剑桥初中物理关于声与光的考试，高频考点集中在五个方面：一是波的三个基本量（波长、频率、振幅）的识别与计算；二是横波与纵波的区别，尤其是声音是纵波、光是横波；三是声音的传播需要介质，真空不能传声；四是反射定律与折射方向的判断，包括画图题；五是声速与光速的应用计算，例如用回声测距离。下面给你三道典型练习题，先自己思考，再对照答案。

In Cambridge Lower Secondary Physics exams on sound and light, the high-frequency topics concentrate on five areas: first, identifying and calculating the three basic quantities of waves (wavelength, frequency and amplitude); second, the difference between transverse and longitudinal waves, especially that sound is longitudinal and light is transverse; third, that sound needs a medium and cannot travel through a vacuum; fourth, the laws of reflection and the direction of refraction, including ray diagram questions; and fifth, calculations using the speed of sound and light, such as measuring distance with echoes. Here are three typical practice questions. Think about them first, then check the answers.

练习题一：一个人站在两座山之间的山谷里，朝一面峭壁喊了一声，3 秒后听到回声。声速取 340 米每秒，峭壁离他有多远？解题思路：声音在 3 秒内走了一个来回，总路程等于 340 乘以 3 等于 1020 米，所以单程距离是 1020 除以 2 等于 510 米。答案：峭壁距离他 510 米。

Practice question 1: A person stands in a valley between two mountains and shouts towards one cliff. He hears the echo 3 seconds later. Taking the speed of sound as 340 m/s, how far away is the cliff? Working: in 3 seconds the sound travels there and back, so the total distance is 340 x 3 = 1020 m, and the one-way distance is 1020 / 2 = 510 m. Answer: the cliff is 510 m away.

练习题二：一根吉他弦每秒振动 440 次，这根弦发出的声音频率是多少？如果另一根弦每秒振动 880 次，哪根弦发出的音调更高？答案：频率就是每秒振动的次数，所以第一根弦的频率是 440 Hz；第二根弦振动更快，频率更高，音调也更高。这正好解释了为什么按住琴弦会改变音高：琴弦变短，振动变快，音调变高。

Practice question 2: A guitar string vibrates 440 times per second. What is the frequency of the sound it produces? If another string vibrates 880 times per second, which string produces the higher pitch? Answer: frequency is the number of vibrations per second, so the first string has a frequency of 440 Hz. The second string vibrates faster, has a higher frequency and therefore a higher pitch. This also explains why pressing down on a guitar string changes the pitch: the string becomes shorter, vibrates faster and the pitch rises.

练习题三：在游泳池边看池底的瓷砖，池子看起来比实际浅。请用光的折射原理解释这一现象。答案：池底反射的光线从水中斜射向空气时，传播速度变快，折射角大于入射角，光线偏离法线弯折。这些光线进入眼睛后，大脑按照光线直线传播的直觉反向延长它们，认为光是从更浅的位置发出的，所以池底看起来比实际位置高，池子就显得浅了。这就是为什么在岸边看水里的鱼，鱼的位置看起来比实际位置更靠近水面，抓鱼时要往更深处伸手。

Practice question 3: Standing by a swimming pool, the tiles at the bottom look closer to the surface than they really are. Explain this using the principle of refraction. Answer: when light from the bottom of the pool travels obliquely from water into air, it speeds up, the angle of refraction is larger than the angle of incidence, and the ray bends away from the normal. When these rays enter your eye, your brain extends them backwards along straight lines, assuming light travels in straight lines, and concludes that the light came from a shallower position. The bottom therefore appears higher than it really is, and the pool looks shallower. This is also why fish in the water appear closer to the surface than they actually are: when you try to catch one by hand, you must reach deeper than where you see it.

Summary | 总结

这篇关于声与光的文章，把剑桥初中物理波动知识的核心内容完整梳理了一遍。我们学习了：波是能量通过振动传递的过程，描述波需要波长、频率和振幅三个量；横波与纵波的区别在于振动方向与传播方向是垂直还是平行；声音由振动产生，是纵波，必须依靠介质传播，真空不能传声，音调由频率决定，响度由振幅决定，人耳的听觉范围是 20 赫兹到 20000 赫兹；回声和超声波的原理是声音的反射，声呐和 B 超都是回声的应用；光是不需要介质的横波，真空中速度约为每秒 30 万千米；光的反射遵循反射定律，平面镜成等大、等距、左右颠倒的虚像；光的折射发生在光速改变时，池水看起来变浅就是折射的杰作；物体呈现的颜色由它反射的光决定，红绿蓝三原色可以混合出各种颜色。掌握这些知识，配合练习题的解题思路，你在声与光这个单元一定能拿到好成绩。

This article has organised the core knowledge of waves in Cambridge Lower Secondary Physics into one complete picture. We have learned that a wave transfers energy through vibrations, and describing a wave needs three quantities: wavelength, frequency and amplitude. Transverse and longitudinal waves differ in whether the vibration is perpendicular or parallel to the direction of travel. Sound is produced by vibrations, is a longitudinal wave, must travel through a medium and cannot pass through a vacuum. Pitch is determined by frequency and loudness by amplitude, and the human ear can hear from 20 Hz to 20000 Hz. Echoes and ultrasound both rely on the reflection of sound, and sonar and medical ultrasound scans are real-world applications. Light is a transverse wave that needs no medium and travels at about 300000 km/s in a vacuum. Reflection follows the laws of reflection, and a plane mirror produces a virtual image that is the same size, equally distant and laterally inverted. Refraction happens when the speed of light changes, and the shallow-looking swimming pool is a perfect example. The colour of an object is the colour it reflects, and red, green and blue can be mixed to create almost any colour. Master these ideas together with the problem-solving steps in the practice section, and you will do well in the sound and light unit.

如果你在声与光、波动或者剑桥初中物理的其他单元还有疑问，欢迎随时联系老师，我们很乐意帮你把每个知识点都弄明白。

If you still have questions about sound and light, waves, or any other unit of Cambridge Lower Secondary Physics, feel free to contact us at any time. We are happy to help you understand every single topic clearly.

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