📚 IGCSE Physics: Diffraction of Light – Key Points Review | IGCSE 物理：光的衍射 考点精讲
Diffraction is a key wave phenomenon that every IGCSE Physics student must understand. When light passes through a narrow slit or around an obstacle, it bends and spreads out, forming a distinctive pattern of bright and dark regions. This article covers all the essential points about diffraction of light as required by the IGCSE syllabus, with clear explanations in both English and Chinese to help you master the topic and excel in your exams.
衍射是每个 IGCSE 物理学生都必须掌握的关键波动现象。当光通过窄缝或绕过障碍物时，会发生弯曲并扩散开来，形成明暗相间的特征图案。本文涵盖了 IGCSE 大纲所要求的光的衍射所有重要考点，并提供清晰的中英双语解释，帮助你彻底掌握这一主题，在考试中取得优异成绩。
1. What is Diffraction? | 什么是衍射？
Diffraction is the spreading out of waves when they pass through a gap or go around an obstacle. This behaviour is a fundamental property of all types of waves, including light, sound and water waves. The amount of diffraction depends on the size of the gap or obstacle relative to the wavelength of the wave. Diffraction provides strong evidence for the wave nature of light.
衍射是指波在穿过缝隙或绕过障碍物时扩散开来的现象。这种行为是所有类型波的基本特性，包括光波、声波和水波。衍射的程度取决于缝隙或障碍物的尺寸与波长的相对大小。衍射为光的波动性提供了有力的证据。
2. Conditions for Noticeable Diffraction | 明显衍射的条件
For diffraction of light to be easily observed, the size of the gap or slit must be very small – comparable to the wavelength of light. Since visible light has wavelengths roughly between 4×10⁻⁷ m and 7×10⁻⁷ m, slits used in a diffraction experiment are typically a fraction of a millimetre wide. If the slit width is much larger than the wavelength, the light passes straight through with almost no spreading, and diffraction effects become negligible.
要想清楚地观察到光的衍射，缝隙或狭缝的尺寸必须非常小——与光的波长相当。可见光的波长大约在 4×10⁻⁷ m 到 7×10⁻⁷ m 之间，因此衍射实验中使用的狭缝宽度通常不到一毫米。如果狭缝宽度远大于波长，光几乎直线通过，几乎不发生扩散，衍射效应就可以忽略不计。
3. Single-Slit Diffraction of Light | 光的单缝衍射
The classic IGCSE demonstration of light diffraction uses a monochromatic light source, such as a laser, and a single narrow slit. When monochromatic light passes through the slit, it diffracts and falls on a screen placed some distance away. Instead of seeing a single bright line matching the slit, we observe a central bright fringe that is much wider than the slit, flanked by alternating dark and bright fringes that become fainter and narrower further away from the centre.
IGCSE 课程中演示光衍射的经典实验使用单色光源（例如激光）和一个窄缝。当单色光穿过狭缝后发生衍射，投射到远处的屏幕上。我们看到的不是一个与狭缝形状对应的亮线，而是一条比狭缝宽得多的中央亮纹，两侧交替分布着暗纹和亮纹，且远离中心的条纹变得更暗、更窄。
4. Explaining the Single-Slit Diffraction Pattern | 单缝衍射图样的解释
The diffraction pattern can be explained using Huygens’ principle. Every point across the slit acts as a source of secondary wavelets. These wavelets interfere with each other. In the forward direction, they arrive in phase and produce a bright central maximum. At certain angles, the path difference between wavelets from opposite ends of the slit leads to destructive interference, creating dark fringes. Constructive interference at other angles forms the subsidiary bright fringes, but these are less intense because the waves only partially reinforce each other.
衍射图样可以用惠更斯原理来解释。狭缝上的每一点都可以看作次级子波的波源，这些子波相互干涉。在正前方，它们同相到达，产生明亮的中央极大。在某些角度上，来自狭缝两端的子波之间存在光程差，导致相消干涉，形成暗纹。在其他角度上的相长干涉则产生次级亮纹，但由于波只部分增强，这些亮纹的强度较弱。
5. Effect of Slit Width on Diffraction | 缝宽对衍射的影响
When the slit width is decreased (becoming narrower), the light waves spread out more after passing through. This causes the central bright fringe to become wider, and the pattern as a whole expands. Conversely, increasing the slit width reduces the amount of spreading, making the central maximum narrower and closer to the geometric shadow of the slit. In the extreme case where the slit is very wide compared to the wavelength, diffraction effects disappear and we simply see a sharp image of the slit.
当狭缝宽度减小时（变得更窄），光通过后扩散得更多。这导致中央亮纹变得更宽，整个图样扩展。相反，增大缝宽会减少扩散程度，使中央极大变窄并更接近狭缝的几何阴影。在狭缝宽度远大于波长的极端情况下，衍射效应消失，我们只会看到狭缝的清晰像。
6. Effect of Wavelength on Diffraction | 波长对衍射的影响
For a fixed slit width, longer wavelengths diffract more than shorter wavelengths. This means that red light (longer λ) produces a wider diffraction pattern than blue light (shorter λ). If we keep the slit width constant and change the colour of the light, we observe that red light creates a broader central maximum and more widely spaced fringes compared to blue light. This dependence on wavelength is crucial for understanding white-light diffraction.
对于固定的缝宽，长波长的光比短波长的光衍射更明显。这意味着红光（λ 较长）产生的衍射图样比蓝光（λ 较短）更宽。如果保持狭缝宽度不变而改变光的颜色，我们会观察到红光的中央极大更宽，条纹间距更大，而蓝光的图样则较为紧凑。这种对波长的依赖对于理解白光衍射至关重要。
7. Diffraction of White Light | 白光的衍射
When white light is used instead of monochromatic light, each constituent colour diffracts by a different amount. The central maximum appears white because all colours overlap there. On either side, however, coloured fringes are observed, with violet (shortest wavelength) appearing closest to the centre and red (longest wavelength) furthest away. This produces a continuous spectrum, similar to dispersion but caused by diffraction rather than refraction. The overlapping of different orders of colours usually means that only the first-order spectra are clearly seen.
当使用白光代替单色光时，每一种组成颜色的光衍射程度不同。中央极大因所有颜色在此重叠而呈现白色。然而，两侧会出现彩色条纹，其中紫光（波长最短）离中心最近，红光（波长最长）离中心最远。这产生了连续光谱，类似于色散，但由衍射而非折射引起。不同级次颜色的重叠通常意味着只能清晰地看到第一级光谱。
8. Diffraction of Water Waves as a Useful Analogy | 水波衍射的有用类比
IGCSE students often first encounter diffraction in the context of water waves in a ripple tank. When straight water waves pass through a gap in a barrier, they spread out in circular arcs if the gap is comparable to the wavelength. If the gap is much wider than the wavelength, the waves continue almost in straight lines with only slight bending at the edges. This behaviour mirrors the diffraction of light exactly, making water waves an excellent visible model for understanding the factors that affect diffraction. Remember that light, water waves and sound all obey the same diffraction principles.
IGCSE 学生通常首先在水波槽实验中接触到衍射。当平直水波通过屏障上的缝隙时，如果缝隙宽度与波长相当，水波会以圆弧形式扩散开来。如果缝隙远宽于波长，水波几乎继续保持直线传播，仅边缘有轻微弯曲。这种行为与光的衍射完全一致，因此水波是理解影响衍射各因素的绝佳可视化模型。记住，光波、水波和声波都遵循相同的衍射原理。
9. Diffraction of Sound Waves | 声波的衍射
Sound waves have wavelengths in the range of a few centimetres to several metres, which are much larger than those of light. As a result, sound diffracts very effectively around everyday objects and through doorways. This is why you can hear someone speaking even when they are around a corner, but you cannot see them. The large wavelength means that ordinary openings and obstacles are often comparable in size to the wavelength, leading to significant diffraction. Comparing light and sound diffraction reinforces the key idea that diffraction is most pronounced when the gap size is similar to the wavelength.
声波的波长在几厘米到几米之间，比光波波长大得多。因此，声波能够非常有效地绕过日常物体并穿过门口发生衍射。这就是为什么你能听到拐角处有人说话，却看不见他们的原因。大的波长意味着普通开口和障碍物的尺寸往往与波长相当，从而产生明显的衍射。对比光与声波的衍射可以强化一个关键概念：当缝隙尺寸与波长相当时，衍射最为显著。
10. Common Misconceptions and Exam Tips | 常见误区与考试提示
One common mistake is to confuse diffraction with refraction or reflection. Diffraction is solely about the bending of waves around obstacles or through gaps, without a change in medium. Another misconception is that a narrower slit makes the pattern narrower; in fact, it makes the central maximum wider. In exam questions, always link the extent of diffraction to the ratio of wavelength to gap size. If asked to draw a diffraction pattern, ensure the central fringe is the brightest and widest, and that subsidiary maxima are much fainter and symmetrically placed. For white light, label the colours correctly with red on the outside and violet on the inside for each side of the central white maximum.
一个常见错误是将衍射与折射或反射混淆。衍射仅仅与波在障碍物周围或穿过缝隙时的弯曲有关，不涉及介质变化。另一个误区是认为缝宽越窄产生的图样越窄；实际上，缝宽变窄会使中央极大变宽。在考试答题时，一定要将衍射程度与波长和缝隙尺寸的比值联系起来。如果要求画出衍射图样，务必将中央亮纹画得最亮、最宽，并确保次级亮纹较暗且左右对称。对于白光衍射，要正确标出颜色，在中央白色极大两侧，红色在外、紫色在内。
Published by TutorHao | Physics Revision Series | aleveler.com
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