IGCSE Science: Light – Exam-Focused Review | IGCSE 科学：光 考点精讲

📚 IGCSE Science: Light – Exam-Focused Review | IGCSE 科学：光 考点精讲

Light is a fundamental topic in IGCSE Science, bridging physical optics and the electromagnetic spectrum. Understanding how light travels, reflects, refracts and interacts with lenses not only answers direct questions but also underpins explanations about sight, colour and modern technology. This article consolidates the key concepts, common pitfalls and exam-style reasoning you need to master.

光是 IGCSE 科学中的基础课题，连接几何光学与电磁波谱。掌握光的传播、反射、折射以及与透镜的相互作用，不仅能直接解题，也是解释视觉、颜色和现代技术的基础。本文整合了需要掌握的核心概念、常见错误和考试常见的推理方式。

1. Nature of Light | 光的本质

Light is a transverse electromagnetic wave that can travel through a vacuum at a speed of approximately 3.0 × 10⁸ m/s. It does not need a medium, which is why sunlight reaches Earth through space. In IGCSE, we treat light as rays that travel in straight lines when the medium is uniform; this is the ray model of light.

光是一种横波电磁波，能够在真空中以约 3.0 × 10⁸ m/s 的速度传播。它不需要介质，所以太阳光能穿过太空抵达地球。在 IGCSE 中，我们将光视为在均匀介质中沿直线传播的光线，这就是光线的模型。

Light transfers energy. A luminous source emits its own light (e.g. the Sun, a lamp), whereas a non-luminous object reflects light from other sources. Shadows and eclipses are formed because light travels in straight lines and cannot bend around large obstacles.

光传递能量。发光体自身发出光（如太阳、灯泡），而非发光体反射来自其他光源的光。影子和日食月食的形成，正是因为光沿直线传播，不能绕过较大的障碍物。

2. Law of Reflection | 反射定律

When light strikes a smooth surface, it bounces back obeying the law of reflection: the incident ray, the reflected ray and the normal all lie in the same plane, and the angle of incidence (i) equals the angle of reflection (r). Both angles are measured from the normal, an imaginary line perpendicular to the surface at the point of incidence.

当光照射到光滑表面时，会遵循反射定律反弹：入射线、反射线和法线位于同一平面内，且入射角 (i) 等于反射角 (r)。两个角均从法线（在入射点处垂直于表面的假想线）量起。

Smooth surfaces produce clear reflections (specular reflection), while rough surfaces cause scattering (diffuse reflection) because the normals at different points point in different directions. Even in diffuse reflection, each individual ray still obeys the law of reflection locally.

光滑表面产生清晰的反射（镜面反射），而粗糙表面引起散射（漫反射），因为不同点的法线指向不同方向。即使发生漫反射，每条光线在局部仍服从反射定律。

3. Images in Plane Mirrors | 平面镜成像

A plane (flat) mirror forms a virtual, upright, laterally inverted image that is the same size as the object and appears to be located as far behind the mirror as the object is in front. The image cannot be projected onto a screen because the light rays only appear to diverge from the image location; they do not actually pass through it.

平面镜形成的像是虚像、正立、左右颠倒、与物体等大，且看起来位于镜后与物距相等的位置。该像无法投影到屏幕上，因为光线只是看起来从像的位置发散，实际上并未通过该处。

To locate the image, draw two rays from a point on the object that strike the mirror, then extend the reflected rays backwards behind the mirror as dotted lines; their intersection gives the image position. This construction is frequently tested in the practical paper.

要确定像的位置，从物体上一点画两条射向镜面的光线，然后将反射光线用虚线反向延长到镜后；它们的交点就是像的位置。这种作图是实验卷的常考内容。

4. Refraction of Light | 光的折射

Refraction occurs when light passes from one transparent medium into another of different optical density, causing a change in speed and, unless the incidence is along the normal, a change in direction. When light enters a denser medium (e.g. from air to glass), it slows down and bends towards the normal; when it enters a less dense medium, it speeds up and bends away from the normal.

当光从一种透明介质进入另一种光学密度不同的介质时，会发生折射，导致速度改变；除非沿法线入射，否则方向也会改变。光进入光密介质（如从空气到玻璃）时速度减小并向法线偏折；进入光疏介质时速度增大并远离法线偏折。

The amount of bending is described by Snell’s law: n = sin i / sin r, where n is the refractive index of the second medium with respect to the first. IGCSE candidates may be asked to calculate refractive indices, critical angles or to predict whether total internal reflection will occur. Remember that normal incidence (i = 0°) produces no deviation.

偏折程度由斯涅尔定律描述：n = sin i / sin r，其中 n 是第二种介质相对于第一种介质的折射率。IGCSE 考生可能被要求计算折射率、临界角，或判断是否发生全内反射。注意垂直入射 (i = 0°) 时不发生偏折。

5. Total Internal Reflection | 全内反射

Total internal reflection (TIR) happens when light travels from a denser to a less dense medium (e.g. from glass to air) and the angle of incidence exceeds the critical angle (c). At the critical angle, the refracted ray runs along the boundary (r = 90°). For angles greater than c, all light is reflected back into the denser medium.

当光从光密介质射向光疏介质（如从玻璃到空气），且入射角大于临界角 (c) 时，发生全内反射 (TIR)。在临界角时，折射光线沿界面传播 (r = 90°)。当入射角大于 c 时，所有光线全部反射回光密介质。

The critical angle is related to refractive index by sin c = 1/n (where n is the refractive index of the denser medium relative to the less dense one). Optical fibres use TIR to transmit light signals over long distances with very little loss. Medical endoscopes and high-speed internet both rely on this principle.

临界角与折射率的关系为 sin c = 1/n（其中 n 是光密介质相对于光疏介质的折射率）。光纤利用全内反射以极低损耗远距离传输光信号。医用内窥镜和高速互联网都依赖这一原理。

6. Converging Lenses | 凸透镜

A converging (convex) lens is thicker in the middle than at the edges. It brings parallel rays of light to a focus at the principal focus. The distance from the centre of the lens to the principal focus is the focal length f. A lens has two principal foci, one on each side.

凸透镜中间厚、边缘薄，能将平行光线会聚到主焦点上。透镜中心到主焦点的距离是焦距 f。一个透镜有两个主焦点，各在透镜一侧。

The type of image formed by a converging lens depends on the object distance u relative to the focal length. When the object is beyond 2F (twice the focal length), the image is real, inverted and smaller. When the object is at 2F, the image is real, inverted and the same size. Between F and 2F, the image is real, inverted and magnified. At F, no clear image is formed. When the object is inside F, the image is virtual, upright and magnified – this is the principle of a magnifying glass.

凸透镜所成像的类型取决于物距 u 与焦距的关系。物体在 2F （两倍焦距）之外时，像是倒立、缩小的实像。在 2F 处时，像是倒立、等大的实像。在 F 和 2F 之间时，像是倒立、放大的实像。在 F 处不成清晰像。物体在 F 以内时，像是正立、放大的虚像 – 这就是放大镜的原理。

7. Ray Diagrams for Lenses | 透镜光路图

To locate an image, draw at least two of three standard rays from the top of the object: (1) a ray parallel to the principal axis, which after refraction passes through the principal focus on the other side; (2) a ray passing through the centre of the lens, which continues undeviated; (3) a ray passing through the principal focus on the object side, which emerges parallel to the axis. The intersection of the refracted rays (or their backward extensions for a virtual image) gives the position of the image.

要确定像的位置，从物体顶端至少画三条标准光线中的两条：(1) 平行于主光轴的光线，折射后通过对侧的主焦点；(2) 通过透镜中心的光线，方向不变；(3) 指向物体侧主焦点的光线，折射后平行于主光轴射出。折射光线（或虚像的反向延长线）的交点就是像的位置。

IGCSE examiners expect accurate ray diagrams drawn with a ruler, with arrows showing direction, and virtual rays drawn as dashed lines. Label the object, image, lens, principal axis and foci clearly. Describe the image using three words: real/virtual, upright/inverted, and magnified/diminished/same size.

IGCSE 考官期望用直尺精确画图，箭头标出方向，虚像用虚线表示。清晰标注物体、像、透镜、主光轴和焦点。用三个词描述像的性质：实像/虚像、正立/倒立、放大/缩小/等大。

8. Dispersion and Colour | 色散与颜色

White light is a mixture of all the colours of the visible spectrum. When white light passes through a triangular glass prism, it is refracted twice and dispersed into a continuous spectrum of colours – from red (longest wavelength, least refracted) to violet (shortest wavelength, most refracted). This is dispersion.

白光是可见光谱中所有颜色的混合。当白光通过三棱镜时，经过两次折射后被色散成连续光谱 – 从红色（波长最长，偏折最小）到紫色（波长最短，偏折最大）。这就是色散。

The order of colours can be remembered as ROYGBIV: Red, Orange, Yellow, Green, Blue, Indigo, Violet. A rainbow is a natural example of dispersion, caused by water droplets in the atmosphere acting like tiny prisms.

颜色顺序可以记作 ROYGBIV：红、橙、黄、绿、蓝、靛、紫。彩虹是色散的自然实例，由大气中的水滴如同微小棱镜一样产生。

Colour filters transmit only certain wavelengths. An object appears red because it reflects red light and absorbs other colours; in red light a pure blue object would appear black. The three primary colours of light are red, green and blue – combining them in different ratios produces all other colours; mixing all three in equal intensity gives white light.

滤色片只允许特定波长的光透过。物体呈现红色是因为它反射红光并吸收其他颜色；在红光下，纯蓝色物体会呈黑色。光的三原色是红、绿、蓝 – 以不同比例组合可产生所有其他颜色；等强度混合三原色得到白光。

9. Electromagnetic Spectrum | 电磁波谱

Light is just one small part of the electromagnetic (EM) spectrum, a family of transverse waves that all travel at the same speed in a vacuum and differ only in wavelength and frequency. The main regions, in order of increasing frequency (and decreasing wavelength), are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.

光只是电磁波谱的一小部分。电磁波是一类横波，在真空中传播速度相同，区别仅在于波长和频率。按频率递增（波长递减）的顺序，主要波段为：无线电波、微波、红外线、可见光、紫外线、X 射线和伽马射线。

Region 波段	Wavelength range 波长范围（大约）	Typical uses and hazards 典型用途与危害
Radio waves 无线电波	> 0.1 m	Broadcasting, communications; no known hazard at typical exposure 广播、通信；典型暴露下无已知危害
Microwaves 微波	1 mm – 0.1 m	Cooking, satellite links, radar; internal heating of body tissue 烹饪、卫星通讯、雷达；对人体组织内部加热
Infrared 红外线	700 nm – 1 mm	Thermal imaging, remote controls, heaters; skin burns 热成像、遥控器、加热器；皮肤灼伤
Visible light 可见光	~400 – 700 nm	Seeing, photography; bright sources can damage eyes 视觉、摄影；强光可损伤眼睛
Ultraviolet 紫外线	10 – 400 nm	Sun tanning, fluorescence, sterilisation; skin cancer, eye damage 日光浴、荧光、消毒；皮肤癌、眼损伤
X-rays X 射线	0.01 – 10 nm	Medical imaging, security; cancer, cell mutation 医学成像、安检；致癌、细胞突变
Gamma rays 伽马射线	< 0.01 nm	Sterilising medical equipment, cancer treatment; severe cell damage 医疗器械消毒、癌症治疗；严重细胞损伤

Higher frequency EM waves carry more energy and are generally more ionising. Gamma rays and X-rays have enough energy to ionise atoms, making them hazardous to living tissue. Understanding these properties is crucial for applications and safety precautions.

频率越高的电磁波携带的能量越多，通常也更具电离性。伽马射线和 X 射线有足够能量使原子电离，对活体组织造成危害。理解这些性质对实际应用和安全防护至关重要。

10. Exam Tips and Common Pitfalls | 考试技巧与常见误区

Always state the law of reflection fully: mention the incident ray, reflected ray, normal, plane and equality of angles. For refraction, many students forget to label the normal or measure angles from the normal; remember to write Snell’s law as n = sin i / sin r and use the correct n for the relevant boundary.

务必完整表述反射定律：提到入射线、反射线、法线、共面及角度相等。在折射问题中，许多学生忘记画法线或从法线起量角度；记住写出斯涅尔定律 n = sin i / sin r，并针对边界选用正确的折射率。

When drawing lens ray diagrams, a common error is to use a single line instead of showing the change in direction at the centre; the ray through the centre is straight, but the other rays must bend at the lens plane. Diagrams must be neat, with a sharp pencil and ruler. Always describe the image using three characteristics.

在画透镜光路图时，常见错误是只用单线表示通过中心的光线而未体现方向变化；通过中心的光线是直线，但其他光线必须在透镜平面处偏折。作图须整洁，用尖铅笔和直尺。始终用三个特征描述像的性质。

For colour and dispersion, do not confuse the primary colours of light (red, green, blue) with those of pigments (cyan, magenta, yellow). In an exam question about coloured lights shining on objects, apply the principle of subtraction: an object only reflects its own colour; all other colours are absorbed.

对于颜色和色散，不要将光的三原色（红、绿、蓝）与颜料的三原色（青、品红、黄）混淆。遇到彩色光照射物体的考题时，应用减法原理：物体只反射其本身颜色，其他颜色均被吸收。

When answering questions on electromagnetic waves, link the order of the spectrum to uses and dangers. A typical comparative question might ask why infrared is used for thermal imaging while X-rays are used for bone scans. Connect wavelength and penetration ability to the application.

回答电磁波相关问题时，将谱序与用途和危害联系起来。典型的比较题可能问为什么红外线用于热成像而 X 射线用于骨骼扫描。要把波长、穿透能力与实际应用挂钩。

Finally, show working clearly in calculations. Whether you are finding the critical angle from a refractive index or image distance from the lens formula (if included in your syllabus), present the formula, substitute values and state the answer with a correct unit. Consistency in units and checking for ‘real or virtual’ can turn a good answer into a full-mark one.

最后，计算题要清晰展示步骤。无论是由折射率求临界角，还是用透镜公式（若大纲包含）求像距，都要写出公式、代入数值并给出带正确单位的答案。单位一致、确认“实像还是虚像”往往能让原本不错的答案变为满分答案。

---

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）