IGCSE WJEC Physics: Diffraction of Light 考点精讲 | IGCSE WJEC 物理：光的衍射考点精讲

📚 IGCSE WJEC Physics: Diffraction of Light 考点精讲 | IGCSE WJEC 物理：光的衍射考点精讲

Diffraction is one of the key wave properties you must master for the IGCSE WJEC Physics exam. When light passes through a narrow slit or around an edge, it bends and spreads out – a phenomenon known as diffraction. Understanding how and why this happens not only helps you explain everyday optical effects but also provides strong evidence for the wave nature of light. In this revision guide, we will break down all the essential points about diffraction of light, from the basic principles to common exam pitfalls.

衍射是IGCSE WJEC物理考试必须掌握的关键波动性质之一。当光通过一个狭窄的缝隙或绕过障碍物的边缘时，它会弯曲并扩散开来——这就是衍射。理解这一现象如何发生以及为何发生，不仅能够帮助你解释日常的光学效应，也为光的波动本性提供了有力的证据。在本考点精讲中，我们将拆解光的衍射所有核心要点，从基本原理到常见的考试误区。

1. What is Diffraction? | 什么是衍射？

Diffraction is the spreading out of waves when they pass through a gap or around an obstacle. This effect is most noticeable when the size of the gap or obstacle is comparable to the wavelength of the wave. For example, water waves spread out in a circular pattern after passing through a narrow opening in a harbour wall. In the same way, light waves diffract when they encounter a very narrow slit, producing a pattern of bright and dark regions on a screen.

衍射是波在穿过缝隙或遇到障碍物时发生扩散的现象。当缝隙或障碍物的尺寸与波的波长相当时，这种效应最为明显。例如，水波通过港口防波堤的狭窄开口后会以圆形图案扩散开来。同样，光波在遇到非常狭窄的缝隙时也会发生衍射，在屏幕上产生明暗相间的图案。

For light, diffraction is not usually seen in everyday life because the wavelength of visible light is extremely small (around 5×10⁻⁷ m). Only when the gap width is around this size will obvious diffraction occur. A single human hair, for instance, can cause laser light to diffract and produce a pattern of light and dark fringes.

对于光来说，日常生活中通常看不到衍射现象，因为可见光的波长极小（大约在5×10⁻⁷ m）。只有当缝隙宽度接近这个尺寸时，才会出现明显的衍射。例如，一根人的头发就能使激光产生衍射，形成明暗条纹。

2. Huygens’ Principle and Wavefronts | 惠更斯原理与波前

To understand diffraction, we often use Huygens’ principle. Every point on a wavefront acts as a source of secondary wavelets that spread out in all directions. When a wavefront hits a barrier with a narrow slit, only the wavelets from the part of the wavefront inside the slit can continue forward. These wavelets then spread out on the far side, causing the wave to bend around the corners of the slit.

为了理解衍射，我们经常运用惠更斯原理。波前上的每一点都可以看作是向各个方向发出子波的波源。当波前碰到带有窄缝的障碍物时，只有波前上位于缝隙内部的那部分子波能够继续向前传播。这些子波在缝隙后方扩散开来，导致波在缝隙的拐角处发生弯曲。

When the slit width is much larger than the wavelength, the spreading effect is minimal because the central part of the wavefront passes straight through while only the edges diffract slightly. When the slit width is approximately equal to the wavelength, the entire wavefront emerging from the slit behaves like a single point source, giving rise to a semicircular wave pattern.

当缝宽远大于波长时，扩展效应很小，因为波前中央部分直接通过，只有边缘部分发生轻微的衍射。当缝宽与波长近似相等时，从缝隙发出的整个波前就像一个点波源，产生半圆形的波阵面图样。

3. Diffraction of Light: Single Slit Experiment | 光的衍射：单缝实验

A classic experiment to observe the diffraction of light uses a monochromatic light source (such as a laser) and a narrow single slit. The laser beam is directed onto a screen through the slit, and the resulting pattern is observed. The setup must be in a darkened room to clearly see the fringes.

一个观察光的衍射的经典实验使用单色光源（例如激光）和一个狭窄的单缝。激光束通过狭缝照射到屏幕上，观察产生的图案。实验装置需要放置在黑暗的房间中，以便清晰地看到条纹。

Instead of seeing just a single bright line, you will observe a central bright band (called the central maximum) that is wider and much brighter than the rest of the pattern. On either side, there are alternating dark and bright fringes. The bright fringes are called secondary maxima, and their intensity decreases as you move further away from the centre.

你不会只看到一条单一的亮线，而是会观察到一个中央亮带（称为中央极大），它比图案中的其他部分更宽、更亮。在中央极大两侧，交替出现暗纹和亮纹。这些亮纹称为次级极大，它们的强度随着远离中心而逐渐减弱。

4. Formation of Central Maximum and Fringes | 中央极大与明暗条纹的形成

The single slit diffraction pattern arises from the superposition of light waves coming from different points across the slit. Light waves that travel straight ahead from all points in the slit arrive in phase at the centre of the screen, producing constructive interference and forming the bright central maximum.

单缝衍射图样是由来自狭缝上不同点的光波叠加而形成的。从狭缝上所有点沿直线向前传播的光波到达屏幕中心时相位相同，产生相长干涉，从而形成明亮的中央极大。

At other angles, waves from one half of the slit can be exactly out of phase with waves from the other half. If the path difference between the two sets of waves is a half-wavelength (λ/2), they cancel out, creating a dark fringe. For the first dark fringe, the condition is:

a sin θ = λ

在其他角度，来自狭缝一半的光波可能与来自另一半的光波恰好反相。如果这两组波之间的光程差是半个波长(λ/2)，它们就会相互抵消，形成暗纹。第一暗纹的条件是：a sin θ = λ。

The subsequent dark fringes occur at positions where the slit can be divided into an even number of equal regions, each pair cancelling out. Although you do not need to use the equation in the IGCSE exam, knowing that the pattern results from constructive and destructive interference helps you describe the effect correctly.

随后的暗纹出现在狭缝可以被分成偶数个相等区域的那些角度位置，每个配对区域相互抵消。虽然IGCSE考试不需要你使用该公式，但知道图样是相长干涉和相消干涉的结果，能帮助你正确描述衍射现象。

5. Effect of Slit Width on Diffraction | 缝宽对衍射的影响

The width of the single slit dramatically affects the diffraction pattern. When the slit is made narrower, the central maximum becomes wider and the whole pattern spreads out more. Conversely, if the slit is widened, the central maximum becomes narrower and the fringes are more closely packed.

单缝的宽度对衍射图样有显著影响。当缝变得更窄时，中央极大变得更宽，整个图案扩展得更大。相反，如果缝隙变宽，中央极大变窄，条纹排列得更加紧密。

Slit Width (a)	Diffraction Pattern
Very narrow (a ≈ λ)	Very wide central maximum; well-separated fringes; very obvious diffraction
Moderately narrow	Central maximum clearly visible; some spreading of light
Wide (a >> λ)	Central maximum almost the width of the slit image; minimal diffraction

This relationship can be summed up: the narrower the slit relative to the wavelength, the greater the diffraction. In the exam, you may be asked to sketch the change in pattern when the slit width is altered.

这种关系可以总结为：相对于波长，缝隙越窄，衍射越明显。在考试中，你可能会被要求画出缝宽改变时图样的变化。

6. Effect of Wavelength on Diffraction | 波长对衍射的影响

The wavelength of light also plays a crucial role. For a fixed slit width, longer wavelengths diffract more than shorter wavelengths. This means that red light, which has a longer wavelength than blue light, will produce a wider central maximum and more spread-out fringes.

波长也起着至关重要的作用。对于固定的缝宽，波长越长，衍射越显著。这意味着波长比蓝光更长的红光会产生更宽的中央极大和更分散的条纹。

If you replace a red laser with a green laser (shorter wavelength) while keeping the slit width unchanged, the diffraction pattern becomes narrower. The central maximum shrinks, and the dark fringes move closer together. This is a popular topic in data-response questions where a graph of intensity against position is provided.

如果在保持缝宽不变的情况下，将红色激光换成绿色激光（波长更短），衍射图样将变得更窄。中央极大缩小，暗纹相互靠得更近。这是在提供强度-位置关系图的数据分析题中常见的一个考点。

7. White Light Diffraction | 白光衍射

So far we have considered monochromatic light. When white light is passed through a single slit, the diffraction pattern becomes more colourful. The central maximum is white because all wavelengths overlap constructively at the centre. However, on either side, the positions of the dark and bright fringes depend on wavelength, causing the colours to separate.

到目前为止，我们考虑的都是单色光。当白光通过单缝时，衍射图样变得更加多彩。中央极大是白色的，因为所有波长的光在中心处重叠并产生相长干涉。然而，在两侧，暗纹和亮纹的位置取决于波长，导致颜色分离。

For each fringe order (except the central one), the longer-wavelength red light will appear further from the centre than the shorter-wavelength violet light. This results in a spectrum-like pattern, with the inner edge of each bright fringe being violet and the outer edge being red. This effect confirms that white light is made up of a range of colours and that each colour has a different wavelength.

对于每一级条纹（中央除外），长波长的红光会比短波长的紫光出现在距离中心更远的位置。由此产生类似光谱的图案，每个亮纹的内边缘为紫色，外边缘为红色。这一效应证实了白光由一系列颜色组成，且每种颜色具有不同的波长。

8. Diffraction Proves the Wave Nature of Light | 衍射证明光的波动性

Historically, the observation of diffraction was critical in establishing that light behaves as a wave. Newton’s particle theory of light could not explain why light bends around obstacles; it predicted sharp shadows. Diffraction, along with interference, provided strong evidence for Huygens’ wave theory.

从历史上看，衍射的观察对于确立光的波动行为至关重要。牛顿的光的微粒说无法解释光为何会绕过障碍物弯曲；它预测的是清晰的阴影。衍射与干涉一起，为惠更斯的波动说提供了强有力的证据。

In IGCSE Physics, you are often asked to explain how diffraction supports the wave model. The key point is that only waves can diffract – particles would simply travel in straight lines and not spread out after a narrow gap. Therefore, the observation of a diffraction pattern when light passes through a narrow slit is conclusive proof that light exhibits wave-like properties.

在IGCSE物理中，你经常会被要求解释衍射如何支持波动模型。关键点是：只有波才会发生衍射——粒子只会沿直线运动，不会在穿过窄缝后扩散开来。因此，当光通过窄缝时观察到衍射图样，就是光展现波动性质的决定性证据。

9. Diffraction vs Interference | 衍射与干涉的区别

Students sometimes confuse diffraction with interference. It is important to clarify the distinction. Diffraction is the bending and spreading of waves around an obstacle or through an aperture. Interference is the superposition of two or more coherent waves, leading to regions of constructive and destructive combination.

学生有时会将衍射和干涉混淆。明确两者的区别很重要。衍射是波在障碍物周围或通过孔缝时的弯曲和扩散。干涉是两个或多个相干波的叠加，导致相长和相消的区域。

In a single slit diffraction pattern, both phenomena are present: the light waves diffract as they pass through the slit, and then the diffracted waves interfere with one another to produce the bright and dark fringes. However, a single slit relies on wavelets from different parts of the same slit, whereas double-slit interference uses two separate coherent sources.

在单缝衍射图样中，两种现象并存：光波通过缝隙时发生衍射，然后衍射波相互干涉，产生明暗条纹。然而，单缝依赖于来自同一狭缝不同部分的子波，而双缝干涉使用的是两个独立的相干源。

Single slit pattern: broad central maximum, fringes decrease in intensity away from centre.
Double slit pattern: equally spaced, equal-intensity fringes (ideally) with a modulating diffraction envelope.

在考试中，根据条纹间距和强度分布来区分单缝衍射和双缝干涉图样是常见的技能。

10. Typical Exam Questions and Answers | 典型考题与答案

Let’s look at a typical WJEC-style question: “Describe and explain the effect on the diffraction pattern when the single slit width is reduced.” A model answer would state: “When the slit width is reduced, the amount of diffraction increases. This means the central bright fringe becomes wider and the fringes are more spread out. This occurs because the slit width becomes closer to the wavelength of light, allowing the wave to spread more as it passes through.”

我们来看一道典型的WJEC风格题目：“描述并解释单缝宽度减小时对衍射图样的影响。”标准答案可以这样说：“当缝宽减小时，衍射程度增加。这意味着中央亮纹变宽，条纹更加分散。这是因为缝宽变得更接近光的波长，使得光波在穿过时扩散得更多。”

Another common question is: “A student shines a red laser through a single slit and observes a pattern on a screen. She then replaces the red laser with a blue laser of the same intensity. State two differences in the pattern observed.” The answer: “The blue pattern will have a narrower central maximum; the fringes will be closer together (or less spread out).”

另一道常见题目是：“一名学生用红色激光照射单缝，在屏幕上观察到图样。然后她用强度相同的蓝色激光替换了红色激光。说出观察到的图样上的两个不同之处。”答案是：“蓝色图样的中央极大更窄；条纹靠得更近（或扩展程度更小）。”

11. Common Misconceptions | 常见误区

Many students believe that diffraction only occurs with slits, but it can also happen at sharp edges. For example, a razor blade edge will cause light to diffract slightly, blurring the shadow. Another misconception is that a narrower slit always increases the brightness of the pattern. In reality, while a narrower slit increases diffraction, it also reduces the amount of light passing through, so the overall intensity drops.

许多学生认为衍射只会发生在狭缝中，但实际上在尖锐边缘处也会发生。例如，剃须刀片边缘会导致光发生轻微的衍射，使阴影变得模糊。另一个误区是认为更窄的缝总是会增加图样的亮度。实际上，虽然更窄的缝增加了衍射，但也减少了通过的光量，因此整体强度会下降。

Also, do not mix up the central maximum of a single slit diffraction with the equally-spaced interference fringes from a double slit. Remember: single slit → non-uniform spacing and decreasing intensity; double slit → uniform spacing and equal intensity (without diffraction envelope). Being able to sketch these patterns clearly is vital for the exam.

另外，不要把单缝衍射的中央极大与双缝干涉的等距条纹混淆。记住：单缝→非等距且强度递减；双缝→等距且强度相等（不考虑衍射包络）。能够清晰地画出这些图样对考试至关重要。

12. Summary and Revision Tips | 总结与复习技巧

In summary, diffraction of light is observed when light passes through a very narrow slit, producing a central bright band with alternating dark and bright fringes. The degree of spreading increases with longer wavelength and narrower slit width. Diffraction provides compelling evidence for the wave nature of light and should be clearly distinguished from interference.

总之，光通过非常窄的狭缝时会发生衍射，产生一个中央亮带以及交替出现的明暗条纹。扩散程度随着波长变长和缝宽变窄而增加。衍射为光的波动本性提供了令人信服的证据，并且应该与干涉明确区分开来。

Practice drawing the intensity vs. position graph for single slit diffraction.
Be ready to compare patterns for different colours and slit widths.
Learn how to use a laser safely in an experiment to observe diffraction.
Remember that diffraction works for all waves, including sound and water waves, and the same principles apply.

复习时，多练习绘制单缝衍射的强度-位置关系图；准备好比较不同颜色和不同缝宽的图样；了解如何在实验中安全使用激光来观察衍射；记住衍射适用于所有波，包括声波和水波，并且相同的原理也适用。

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IGCSE WJEC Physics: Diffraction of Light 考点精讲 | IGCSE WJEC 物理：光的衍射 考点精讲