IGCSE Physics: A Complete Guide to Light Waves | IGCSE物理：光波知识精讲

📚 IGCSE Physics: A Complete Guide to Light Waves | IGCSE物理：光波知识精讲

Light waves are one of the most fascinating and frequently tested topics in IGCSE Physics. This guide covers the essential concepts required for the Edexcel syllabus, including reflection, refraction, diffraction, colour, and lenses, with clear explanations and worked examples.

光波是IGCSE物理中最迷人且最常考的主题之一。本指南涵盖Edexcel考纲所需的核心概念，包括反射、折射、衍射、颜色和透镜，并提供清晰的解释和例题演示。

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1. The Nature of Light | 光的本质

Light is a form of electromagnetic radiation that travels as transverse waves. It does not require a medium to propagate, meaning it can travel through a vacuum at a speed of approximately 3 × 10⁸ m/s.

光是一种以横波形式传播的电磁辐射。它不需要介质即可传播，这意味着它可以以约3 × 10⁸ m/s的速度在真空中穿行。

• Light waves have both electric and magnetic field components oscillating perpendicular to the direction of travel.

  光波具有电场和磁场分量，二者均在与传播方向垂直的方向上振荡。

• The wavelength of visible light ranges from approximately 400 nm (violet) to 700 nm (red).

  可见光的波长范围约为400 nm（紫光）至700 nm（红光）。

• Light exhibits wave properties such as reflection, refraction, diffraction, and interference.

  光表现出波的特性，如反射、折射、衍射和干涉。

In the wave model, colour is determined by frequency. Higher frequency corresponds to shorter wavelength and higher energy. For example, blue light has a higher frequency than red light.

在波动模型中，颜色由频率决定。频率越高，波长越短，能量越大。例如，蓝光的频率高于红光。

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2. The Electromagnetic Spectrum | 电磁波谱

Light is part of the electromagnetic spectrum, which includes all types of electromagnetic radiation arranged by frequency and wavelength. The visible spectrum is only a small portion of the entire range.

光是电磁波谱的一部分，电磁波谱包含按频率和波长排列的所有类型的电磁辐射。可见光谱只是整个范围中很小的一部分。

Region | 区域	Wavelength Range | 波长范围	Typical Use | 典型用途
Radio waves | 无线电波	> 0.1 m	Communication | 通信
Microwaves | 微波	1 mm – 0.1 m	Cooking, radar | 烹饪、雷达
Infrared | 红外线	700 nm – 1 mm	Thermal imaging | 热成像
Visible light | 可见光	400 – 700 nm	Vision | 视觉
Ultraviolet | 紫外线	10 – 400 nm	Sterilisation | 消毒
X-rays | X射线	0.01 – 10 nm	Medical imaging | 医学成像
Gamma rays | γ射线	< 0.01 nm	Cancer treatment | 癌症治疗

All electromagnetic waves travel at the same speed in a vacuum, but their differing frequencies and wavelengths allow them to interact with matter differently.

所有电磁波在真空中以相同的速度传播，但它们不同的频率和波长使其与物质的相互作用方式各不相同。

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3. Reflection of Light | 光的反射

Reflection occurs when light bounces off a surface. The law of reflection states that the angle of incidence equals the angle of reflection, measured from the normal — an imaginary line perpendicular to the surface at the point of incidence.

反射是光从表面弹回的现象。反射定律指出：入射角等于反射角，二者均以法线为基准测量——法线是在入射点处垂直于表面的假想线。

Angle of incidence i = Angle of reflection r

Two types of reflection are distinguished in the IGCSE syllabus:

IGCSE考纲区分两种类型的反射：

• Regular reflection — occurs on smooth surfaces such as mirrors, producing a clear image because all reflected rays remain parallel.

  镜面反射——发生在如镜子等光滑表面上，所有反射光线保持平行，因此形成清晰的像。

• Diffuse reflection — occurs on rough surfaces such as paper or walls. Reflected rays scatter in different directions, so no clear image forms, but the surface becomes visible from multiple angles.

  漫反射——发生在如纸张或墙壁等粗糙表面上。反射光线向各个方向散射，因此不形成清晰的像，但表面从多个角度都能被看见。

For plane mirrors, the image formed is virtual, upright, laterally inverted (left-right reversed), and the same size as the object. The image distance equals the object distance.

对于平面镜，所成的像是虚像、正立、左右颠倒（镜像反转），且与物体等大。像距等于物距。

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4. Refraction of Light | 光的折射

Refraction is the bending of light as it passes from one transparent medium to another. This occurs because the speed of light changes when the medium changes. When light enters a denser medium, it slows down and bends towards the normal; when it enters a less dense medium, it speeds up and bends away from the normal.

折射是光从一种透明介质进入另一种透明介质时发生的偏折。这是因为光速随介质改变而变化。当光进入密度更大的介质时，速度减慢并向法线方向偏折；当光进入密度较小的介质时，速度加快并远离法线方向偏折。

The refractive index n of a medium is defined as:

介质的折射率n定义为：

n = sin i ⁄ sin r = c ⁄ v

where i is the angle of incidence in a vacuum (or air), r is the angle of refraction in the medium, c is the speed of light in a vacuum, and v is the speed of light in the medium.

其中i是真空（或空气）中的入射角，r是介质中的折射角，c是真空中的光速，v是光在介质中的速度。

Worked example: A ray of light strikes a glass block (n = 1.5) at an angle of 30° to the normal. Calculate the angle of refraction.

例题：一束光以30°入射角射向玻璃块（n = 1.5）。计算折射角。

n = sin i ⁄ sin r → 1.5 = sin 30° ⁄ sin r → sin r = 0.5 ⁄ 1.5 = 0.333 → r = sin⁻¹(0.333) ≈ 19.5°

When light passes through a rectangular glass block, it emerges parallel to the incident ray but laterally displaced. This is often tested in ray diagram questions.

当光穿过矩形玻璃块时，出射光线与入射光线平行但发生了侧向位移。这在光路图题中经常出现。

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5. Total Internal Reflection | 全内反射

Total internal reflection occurs when light travels from a denser medium to a less dense medium at an angle greater than the critical angle. At this point, all light is reflected back into the denser medium.

全内反射发生在光从密度较大的介质射向密度较小的介质，且入射角大于临界角时。此时，所有光都被反射回密度较大的介质中。

The critical angle c is given by:

临界角c由下式给出：

sin c = 1 ⁄ n

For glass with n = 1.5, the critical angle is approximately 41.8°. Beyond this angle, no refraction occurs — only reflection.

对于n = 1.5的玻璃，临界角约为41.8°。超过此角度，不再发生折射——只有反射。

Applications of total internal reflection include:

全内反射的应用包括：

• Optical fibres — light signals travel through thin glass fibres by repeated total internal reflection, enabling high-speed internet and endoscopy.

  光纤——光信号通过反复的全内反射在细玻璃纤维中传播，实现高速互联网和内窥镜检查。

• Prisms in binoculars — right-angled prisms reflect light by 90° or 180° more efficiently than mirrors.

  双筒望远镜中的棱镜——直角棱镜比镜子更高效地将光反射90°或180°。

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6. Diffraction of Light | 光的衍射

Diffraction is the spreading of waves as they pass through a gap or around an obstacle. For light, diffraction is observable when the gap is comparable in size to the wavelength of light — typically very small openings or slits.

衍射是波通过狭缝或绕过障碍物时发生的展宽现象。对于光来说，当狭缝尺寸与光的波长相当——通常是非常小的开口或狭缝——才能观察到衍射。

In IGCSE Physics, you should be able to describe:

在IGCSE物理中，你应该能够描述：

• When light passes through a narrow slit, the pattern on a screen shows a central bright fringe with dimmer fringes on either side.

  当光通过窄缝时，屏幕上的图样显示中央亮条纹，两侧有较暗的条纹。

• The narrower the slit, the greater the spreading of light — diffraction is more pronounced.

  狭缝越窄，光的展宽越明显——衍射越显著。

• Diffraction provides strong evidence for the wave nature of light.

  衍射为光的波动性提供了有力证据。

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7. Dispersion of White Light | 白光色散

Dispersion is the splitting of white light into its component colours when it passes through a prism. This happens because different colours (frequencies) of light have different refractive indices in glass — a phenomenon known as dispersion.

色散是白光通过棱镜时被分解为其组成颜色的现象。这是因为不同颜色（频率）的光在玻璃中具有不同的折射率——该现象称为色散。

White light consists of seven main colours in sequence: red, orange, yellow, green, blue, indigo, and violet (ROYGBIV). Red light deviates the least because it has the lowest refractive index, while violet light deviates the most.

白光按顺序包含七种主要颜色：红、橙、黄、绿、蓝、靛、紫（ROYGBIV）。红光的偏折最小，因为其折射率最低；紫光的偏折最大。

A rainbow is a natural example of dispersion, where sunlight is refracted, dispersed, and internally reflected by raindrops.

彩虹是色散的自然实例，阳光在雨滴中被折射、色散并发生内反射。

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8. Colour and Filters | 颜色与滤光片

The colour of an object depends on how it interacts with incident light:

物体的颜色取决于它如何与人射光相互作用：

• Opaque objects — the colour of an opaque object is the colour of light it reflects. A red apple absorbs all colours except red, which it reflects to our eyes.

  不透明物体——不透明物体的颜色是它所反射的光的颜色。一个红苹果吸收除了红色以外的所有颜色，只将红色反射到我们的眼睛中。

• Transparent objects — the colour of a transparent object is the colour of light it transmits. A blue filter transmits blue light and absorbs all other colours.

  透明物体——透明物体的颜色是它所透射的光的颜色。蓝色滤光片透射蓝光，吸收所有其他颜色。

A common exam question involves predicting the colour of an object under different coloured lights:

一个常见的考题是预测物体在不同颜色光照下的颜色：

Object | 物体	White light | 白光下	Red light | 红光下	Blue light | 蓝光下
Red apple | 红苹果	Red | 红色	Red | 红色	Black | 黑色
Blue book | 蓝书	Blue | 蓝色	Black | 黑色	Blue | 蓝色
White shirt | 白衬衫	White | 白色	Red | 红色	Blue | 蓝色

Remember: a black object absorbs all colours and reflects none. A white object reflects all colours equally.

记住：黑色物体吸收所有颜色，不反射任何颜色。白色物体等比例反射所有颜色。

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9. Lenses | 透镜

A lens is a transparent material, usually glass or plastic, that refracts light to form an image. Two main types of lenses are covered in the IGCSE syllabus:

透镜是由玻璃或塑料等透明材料制成的光学器件，通过折射光线来成像。IGCSE考纲覆盖两种主要类型的透镜：

• Convex (converging) lens — thicker in the middle than at the edges. It converges parallel rays of light to a focal point. Used in magnifying glasses, cameras, and the human eye.

  凸透镜（会聚透镜）——中间厚、边缘薄。它将平行光线会聚到焦点。用于放大镜、照相机和人眼。

• Concave (diverging) lens — thinner in the middle than at the edges. It spreads parallel rays outward as if they came from a virtual focal point. Used in correcting short-sightedness.

  凹透镜（发散透镜）——中间薄、边缘厚。它将平行光线向外发散，仿佛它们来自一个虚焦点。用于矫正近视。

Key terms:

关键术语：

• Principal focus (F): the point on the principal axis where parallel rays converge (convex) or appear to diverge from (concave).

  主焦点（F）：平行光线会聚（凸透镜）或似乎发散的起点（凹透镜）所在的主轴上的点。

• Focal length (f): the distance from the centre of the lens (optical centre) to the principal focus.

  焦距（f）：从透镜中心（光心）到主焦点的距离。

• Object distance (u): distance from the object to the optical centre.

  物距（u）：从物体到光心的距离。

• Image distance (v): distance from the image to the optical centre.

  像距（v）：从像到光心的距离。

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10. Drawing Ray Diagrams | 作光路图

Ray diagrams are essential for determining the position, size, and nature of images formed by lenses. Three principal rays are used:

光路图对于确定透镜所成像的位置、大小和性质至关重要。需使用三条主光线：

• Ray 1: A ray parallel to the principal axis refracts through the focal point on the far side of a convex lens (or appears to come from the focal point on the near side of a concave lens).

  光线1：平行于主轴的光线经凸透镜折射后通过另一侧的焦点（对于凹透镜，则看起来来自同侧焦点）。

• Ray 2: A ray passing through the optical centre continues in a straight line without deviation.

  光线2：通过光心的光线沿直线传播，不发生偏折。

• Ray 3: A ray passing through the focal point before reaching the lens emerges parallel to the principal axis.

  光线3：经过焦点后到达透镜的光线，出射后平行于主轴。

For a convex lens with object beyond 2F, the image is real, inverted, diminished, and located between F and 2F. With object between F and 2F, the image is real, inverted, magnified, and located beyond 2F. With object inside F, the image is virtual, upright, and magnified.

对于凸透镜：当物体位于2F以外时，成实像、倒立、缩小，像位于F与2F之间；当物体位于F与2F之间时，成实像、倒立、放大，像位于2F以外；当物体位于F以内时，成虚像、正立、放大。

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11. The Lens Formula | 透镜公式

The lens formula relates object distance, image distance, and focal length:

透镜公式将物距、像距和焦距联系起来：

1 ⁄ f = 1 ⁄ u + 1 ⁄ v

Conventions: f is positive for convex lenses and negative for concave lenses. v is positive for real images and negative for virtual images.

符号约定：凸透镜f为正，凹透镜f为负。实像v为正，虚像v为负。

Magnification is given by:

放大率由下式给出：

M = image height ⁄ object height = v ⁄ u

Worked example: An object is placed 15 cm from a convex lens of focal length 10 cm. Calculate the image distance.

例题：物体放置在距焦距为10 cm的凸透镜15 cm处。计算像距。

1 ⁄ 10 = 1 ⁄ 15 + 1 ⁄ v → 1 ⁄ v = 1 ⁄ 10 − 1 ⁄ 15 = (3 − 2) ⁄ 30 = 1 ⁄ 30 → v = 30 cm

Since v is positive, the image is real and located 30 cm from the lens on the opposite side.

由于v为正，像是实像，位于透镜另一侧30 cm处。

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12. The Eye and Vision Defects | 眼睛与视力缺陷

The human eye functions like a camera. The cornea and lens focus light onto the retina, where photoreceptor cells detect the image. The ciliary muscles change the shape of the lens to adjust focus — this is called accommodation.

人眼的功能类似照相机。角膜和晶状体将光聚焦在视网膜上，感光细胞在此检测图像。睫状肌通过改变晶状体的形状来调节焦点——这称为调节。

Two common vision defects are tested:

考纲中考查两种常见的视力缺陷：

• Short-sightedness (myopia): the eye focuses light in front of the retina, so distant objects appear blurred. It is corrected with a concave (diverging) lens, which spreads the light slightly before it enters the eye.

  近视（myopia）：眼睛将光聚焦在视网膜前方，因此远处物体看起来模糊。使用凹透镜（发散透镜）矫正，使光线在进入眼睛前略微发散。

• Long-sightedness (hyperopia): the eye focuses light behind the retina, so near objects appear blurred. It is corrected with a convex (converging) lens, which converges the light more strongly before it enters the eye.

  远视（hyperopia）：眼睛将光聚焦在视网膜后方，因此近处物体看起来模糊。使用凸透镜（会聚透镜）矫正，使光线在进入眼睛前更强地会聚。

In ray diagrams, the correction lens must be drawn such that the light entering the eye converges correctly onto the retina. For myopia, the diverging lens causes rays to spread before entering the cornea, pushing the image back to the retina. For hyperopia, the converging lens causes rays to converge earlier, bringing the image forward to the retina.

在光路图中，矫正透镜的绘制必须使进入眼睛的光线正确会聚到视网膜上。对于近视，凹透镜使光线在进入角膜前发散，将像后移回到视网膜上。对于远视，凸透镜使光线更早会聚，将像前移到视网膜上。

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