GCSE Edexcel Physics: Diffraction of Light – Key Points Explained | GCSE Edexcel 物理：光的衍射 考点精讲

📚 GCSE Edexcel Physics: Diffraction of Light – Key Points Explained | GCSE Edexcel 物理：光的衍射 考点精讲

Waves are all around us, and one of their most fascinating behaviours is diffraction. In Edexcel GCSE Physics, understanding how light spreads out after passing through a narrow slit or around an obstacle is a key component of the waves topic. This article breaks down the essential ideas, experimental observations, and exam-ready facts you need to master the diffraction of light.

波无处不在，而衍射是波最迷人的行为之一。在 Edexcel GCSE 物理中，理解光在穿过狭缝或绕过障碍物后如何扩展是波动章节的关键内容。本文将为你拆解核心概念、实验现象以及应对考试所必须掌握的要点，帮助你彻底掌握光的衍射。

1. What is Diffraction? | 什么是衍射？

Diffraction is the spreading out of waves when they pass through a gap or travel around an obstacle. The effect is most noticeable when the size of the gap or obstacle is comparable to the wavelength of the wave. Diffraction is a property that all waves share, including sound, water, and light.

衍射是指波在穿过缝隙或绕过障碍物时向外扩展的现象。当缝隙或障碍物的尺寸与波的波长相近时，衍射效果最为明显。衍射是所有波共有的性质，包括声波、水波和光波。

2. Diffraction of Water Waves (Introduction) | 水波的衍射（引入）

To visualise diffraction, ripple tank experiments with water waves are often used. When straight water waves pass through a wide gap, they mainly continue straight through, with only slight bending at the edges. However, when the gap is narrowed to match the wavelength, the waves spread out in a semicircular pattern on the far side. This same principle applies to light, even though light’s wavelength is much smaller.

为了直观地观察衍射，我们常使用水波槽进行实验。当平直的水波通过较宽的缝隙时，它们基本沿直线传播，仅在边缘有轻微弯曲。但当缝隙缩小到与波长相近时，水波在缝隙的另一侧会以半圆形的形式扩散开来。光的衍射遵循同样的原理，尽管光的波长要小得多。

3. Diffraction of Light: Experimental Setup | 光的衍射：实验装置

To observe the diffraction of light, a coherent light source such as a laser is directed at a single narrow slit. The slit is typically formed by two closely spaced blades or a precision-cut slide. Beyond the slit, a screen is placed to capture the pattern. The room should be darkened to make the faint fringes visible.

为了观察光的衍射，需要一个相干光源（如激光）照射在一条单缝上。狭缝通常由两片靠得很近的刀片或精密刻制的狭缝片构成。在狭缝后方放置一个屏幕以接收图样。实验室需保持黑暗，以便看清微弱的条纹。

A key requirement is that the slit width must be very small – typically a few tenths of a millimetre or less. This is because the wavelength of visible light is around 400–700 nanometres (nm), so the slit needs to be close to this scale for significant diffraction.

关键条件是狭缝必须非常窄——通常为零点几毫米甚至更小。这是因为可见光的波长约为 400–700 纳米，要使衍射显著，狭缝宽度必须接近这一尺度。

4. Single Slit Diffraction Pattern | 单缝衍射图样

When monochromatic light (light of a single colour) passes through a single slit, a characteristic diffraction pattern is formed on the screen. It consists of a bright central band, called the central maximum, flanked by alternating dark and bright fringes that become dimmer and narrower as they move away from the centre.

当单色光（单一颜色的光）穿过单缝时，屏幕上会形成一种特征性的衍射图样。它由一条明亮的中央条纹（中央极大）和两侧交替出现的暗条纹与亮条纹组成。随着与中心的距离增大，亮条纹变得越来越暗、越来越窄。

The central maximum is much wider and brighter than the secondary maxima. The dark fringes occur where the light waves interfere destructively, while the bright fringes are regions of constructive interference resulting from different parts of the wavefront passing through the slit.

中央极大的宽度和亮度远大于次级极大。暗条纹对应光波发生相消干涉的位置，而亮条纹则是通过狭缝的不同部分波前发生相长干涉的结果。

Feature 特征	Description 描述
Central maximum 中央极大	Widest and brightest; twice the width of other bright fringes 最宽最亮；宽度约为其他亮条纹的两倍
Dark fringes 暗条纹	Positions of complete destructive interference 完全相消干涉的位置
Secondary maxima 次级极大	Diminishing intensity outward; much fainter than centre 向外强度递减；远暗于中心

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

The amount of spreading depends on the slit width relative to the wavelength. If the slit width is much larger than the wavelength, the wave passes through with only minimal spreading, giving a sharp image of the slit on the screen. As the slit width decreases and approaches the wavelength, the light spreads out much more, and the diffraction pattern widens.

衍射的扩散程度取决于缝宽与波长的相对大小。若缝宽远大于波长，光波几乎不扩散地通过，在屏幕上形成一个清晰的狭缝像。当缝宽减小并接近波长时，光的扩散显著增强，衍射图样也相应展宽。

When the slit width is smaller than the wavelength, the diffraction pattern becomes so wide that the central maximum covers almost the entire screen, and the secondary maxima become very dim and spread out.

当缝宽小于波长时，衍射图样变得极宽，中央极大几乎覆盖整个屏幕，次级极大则非常暗淡且分布松散。

Slit width a ≈ wavelength λ → maximum diffraction

缝宽 a ≈ 波长 λ → 衍射效果最强

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

For a fixed slit width, longer wavelengths diffract more. Red light (longer wavelength) spreads out more than blue light (shorter wavelength) when passing through the same slit. This means that the central maximum for red light is wider, and the whole pattern is more spread out.

对于固定的缝宽，波长越长，衍射越显著。红光（波长较长）在通过同一狭缝时比蓝光（波长较短）扩散得更开。这意味着红光的中央极大更宽，整个衍射图样也更加分散。

This dependence explains why the diffraction pattern for white light shows coloured fringes. Since white light is a mixture of all visible wavelengths, different colours diffract by different amounts, spreading into a spectrum.

这种依赖关系解释了为什么白光的衍射图样会呈现彩色条纹。由于白光是所有可见波长的混合，不同颜色的光衍射程度不同，从而展开形成光谱。

7. White Light Diffraction | 白光的衍射

When white light passes through a single slit, the central maximum remains white because all wavelengths undergo constructive interference at the centre. However, the surrounding fringes show a rainbow of colours. Violet light (shorter wavelength) appears closer to the centre, and red light (longer wavelength) appears farther out. This creates a continuous spectrum on both sides, with red outermost and violet innermost.

当白光通过单缝时，中央极大仍为白色，因为所有波长的光在中央处均发生相长干涉。然而，两侧的条纹却展现出彩虹般的颜色。紫色光（波长较短）出现在靠近中心的位置，红色光（波长较长）则出现在更外侧。这样就在屏幕两侧形成了连续光谱，红色在外、紫色在内。

It is important not to confuse this with the pattern produced by a prism or a diffraction grating – in a single slit, the spectrum is less pure and the colours overlap more, but the dispersion order from violet to red is the same.

重要的是不要将此图样与棱镜或衍射光栅产生的光谱混淆——在单缝衍射中，光谱的纯度较低，颜色重叠更多，但从紫到红的色散顺序是相同的。

8. Diffraction with a Gap vs. an Obstacle | 缝隙与障碍物的衍射

Diffraction occurs not only when waves pass through a gap, but also when they encounter an obstacle. If light waves hit a small obstacle, they bend around its edges and produce a pattern behind it. A famous example is Poisson’s spot (Arago spot) – a bright spot that appears at the centre of the shadow of a circular object due to diffraction.

衍射不仅发生在波通过缝隙时，当波遇到障碍物时同样会发生。如果光波照射在一个小障碍物上，它们会围绕物体的边缘弯曲，并在其后方产生衍射图样。一个著名的例子是泊松亮斑（阿拉果斑）——由于衍射，在圆形物体的阴影中心会出现一个亮点。

In GCSE Edexcel, the focus is mostly on diffraction through gaps, but understanding that obstacles also cause diffraction helps link to everyday observations, such as the spreading of sound around corners or the blurring of shadows.

在 GCSE Edexcel 考纲中，重点通常放在缝隙衍射上，但理解障碍物也会引起衍射有助于联系日常现象，例如声音绕墙角传播或阴影边缘的模糊。

9. Comparing Diffraction and Interference | 衍射与干涉的比较

Diffraction and interference are closely linked wave phenomena. Diffraction produces the wavefronts that can then overlap and interfere. The single-slit diffraction pattern itself is a result of interference between secondary wavelets from different parts of the same slit (Huygens–Fresnel principle). In exam answers, avoid mixing up single-slit diffraction with double-slit interference.

衍射和干涉是紧密关联的波动现象。衍射产生了可以相互叠加和干涉的波前。单缝衍射图样本身就是来自同一狭缝不同部分的次级子波之间干涉的结果（惠更斯-菲涅耳原理）。在考试作答中，要避免将单缝衍射与双缝干涉混为一谈。

In double-slit experiments, each slit acts as a coherent source, leading to equally spaced bright and dark fringes. In single-slit diffraction, the central maximum is much wider, and the fringes are not equally spaced. This is a frequently tested distinction in Edexcel GCSE Physics.

在双缝实验中，每一条缝都充当一个相干光源，产生等间距的明暗相间条纹。而在单缝衍射中，中央极大远宽于其他条纹，且条纹间距并不相等。这是 Edexcel GCSE 物理中经常考查的区分点。

10. Applications and Everyday Examples | 应用与日常实例

Although light diffraction is often demonstrated in the lab, it has practical applications. Diffraction limits the resolution of optical instruments such as microscopes and telescopes. When looking at very close stars, the telescope’s aperture acts as a circular slit, causing diffraction patterns that can blur the image.

尽管光的衍射多在实验室中演示，但它具有实际应用。衍射限制了显微镜和望远镜等光学仪器的分辨率。当观察非常接近的双星时，望远镜的通光口径就像一个圆形孔隙，产生的衍射图样会使图像变得模糊。

Diffraction gratings – a natural extension – are widely used in spectrometers to analyse light from stars and determine their chemical composition. However, for GCSE, the specification centres on the qualitative description of single-slit diffraction.

衍射光栅作为衍射的自然延伸，被广泛用于光谱仪中，用来分析恒星发出的光并确定其化学成分。不过，对 GCSE 而言，考纲重点在于单缝衍射的定性描述。

Everyday examples include seeing the coloured pattern on the surface of a CD or DVD; the closely spaced tracks act as a reflective diffraction grating. Another is the fuzzy edge of a shadow when the light source is partially blocked.

生活中的例子包括 CD 或 DVD 表面呈现的彩色图样，因为紧密排列的刻录轨道相当于一个反射衍射光栅。另一个例子是光源被部分遮挡时，阴影边缘呈现的模糊现象。

11. Exam Tips and Common Misconceptions | 考试技巧与常见误区

Students often lose marks by confusing diffraction with refraction or reflection. Remember: diffraction is the bending of waves around obstacles or through gaps without a change of medium. Another common error is stating that the wavelength changes during diffraction; the wavelength of light remains the same unless the medium changes. Only the direction of propagation is altered.

考生常因把衍射与折射或反射混淆而失分。请记住：衍射是波在障碍物周围或通过缝隙时的弯曲，并没有改变介质。另一个常见错误是声称衍射过程中波长发生了改变；实际上，只要介质不变，光的波长就保持不变，只有传播方向发生了变化。

Be precise with your language in the exam. For example: “When the slit width is about the same as the wavelength, the light spreads out significantly, producing a wide central bright fringe and a series of dimmer side fringes.” Also, when explaining the central maximum for white light, mention it stays white because all colours overlap constructively there.

考试时用语要精确。例如：“当狭缝宽度与波长相近时，光显著扩散，产生一个宽阔的中央亮条纹和一系列较暗的侧条纹。” 此外，在解释白光中央极大时，要指出它保持白色，因为所有颜色在此处都发生相长叠加。

Diagrams are helpful: sketch a single slit, show the incident plane waves, the curved wavefronts emerging from the slit, and the intensity pattern on the screen. Label central maximum, dark fringe, and secondary maximum.

画图很有帮助：画出单缝，标注入射平面波，从狭缝中出射的弯曲波前，以及屏幕上的强度图样。标明中央极大、暗条纹和次级极大。

12. Summary | 总结

Diffraction of light is a direct consequence of the wave nature of light. It is observed when light passes through a slit of width comparable to its wavelength, producing a characteristic pattern. The amount of spreading depends on the slit width and the wavelength; longer wavelengths diffract more. White light gives a central white maximum with coloured side bands. Being able to describe, sketch, and explain these phenomena, while distinguishing diffraction from interference, is essential for success in Edexcel GCSE Physics.

光的衍射是光具有波动性的直接结果。当光通过与其波长尺寸相当的狭缝时，可观察到特征性的衍射图样。扩散程度取决于缝宽和波长；波长越长，衍射越显著。白光会产生一个白色中央极大和彩色的侧边条纹。能够描述、绘制并解释这些现象，并区分衍射与干涉，是 Edexcel GCSE 物理取得好成绩的关键。

Practise past paper questions on wave properties and diffraction. Pay attention to command words like ‘describe’, ‘explain’, and ‘compare’. Use precise terminology and always link the slit size or wavelength to the observed pattern.

多做关于波动性质和衍射的历年真题。注意‘describe’、‘explain’和‘compare’等指令词。使用准确的科技术语，并总将狭缝尺寸或波长与观察到的图样联系起来。

Published by TutorHao | Physics Revision Series | aleveler.com

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