📚 IGCSE AQA Physics: Diffraction of Light – Key Concepts | IGCSE AQA 物理：光的衍射 考点精讲
Diffraction is the spreading of waves when they pass through a gap or around an obstacle. For light, a type of electromagnetic wave, diffraction provides critical evidence for the wave nature of light. In the IGCSE AQA Physics syllabus, understanding how light bends and produces interference patterns is essential for grasping wave behaviour and tackling exam questions with confidence.
衍射是波通过缝隙或绕过障碍物时发生的扩散现象。光作为一种电磁波，其衍射为光的波动性提供了关键证据。在 IGCSE AQA 物理课程中，理解光如何弯曲并产生干涉图样，对于掌握波动行为和自信应对考题至关重要。
1. What is Diffraction of Light? | 什么是光的衍射？
Diffraction of light occurs when a light wave encounters an obstacle or a slit that is comparable in size to its wavelength. Instead of travelling in a straight line, the wave bends around the corners and spreads out into the region behind the obstacle.
当光波遇到与其波长尺寸相当的障碍物或狭缝时，就会发生光的衍射。光波不再沿直线传播，而是在角落处弯曲并扩散到障碍物后方的区域。
This spreading is most pronounced when the size of the gap or obstacle is roughly equal to the wavelength of the light. For visible light, with wavelengths around 400–700 nm, everyday gaps are far too wide, so diffraction is not obvious unless very fine slits are used.
当缝隙或障碍物的尺寸与光的波长大致相等时，这种扩散最为明显。可见光的波长约为 400–700 nm，日常缝隙都宽得多，因此除非使用非常细的狭缝，否则衍射并不明显。
2. Condition for Noticeable Diffraction | 明显衍射的条件
For a single slit, noticeable diffraction occurs when the slit width w is similar to the wavelength λ of the light. If w is much larger than λ, the wave passes through almost in straight lines with only slight bending at the edges.
对于单缝，当缝宽 w 与光波长 λ 相近时，会发生明显的衍射。若 w 远大于 λ，波几乎沿直线通过，仅在边缘有轻微弯曲。
As the slit becomes narrower, the amount of spreading increases. The central maximum becomes wider, demonstrating that more diffraction occurs when the opening size approaches the wavelength.
当狭缝变窄时，扩散量增大。中央亮纹变得更宽，说明当开口尺寸接近波长时，衍射现象更显著。
3. Single-Slit Diffraction Experiment | 单缝衍射实验
A simple demonstration uses a laser shone through a narrow adjustable slit onto a screen. The slit width is reduced until a clear pattern of bright and dark fringes appears on the screen, showing light spreading into the geometric shadow.
一个简单的演示是让激光穿过可调节的窄缝投射到屏幕上。逐渐减小缝宽，直到屏幕上出现清晰的明暗条纹花样，显示光扩散到了几何阴影区。
The pattern consists of a broad central bright fringe, flanked by alternating dark and secondary bright fringes on both sides. This cannot be explained by geometrical optics and requires the wave model.
该图样由一个宽阔的中央亮纹和两侧交替出现的暗纹与次级亮纹组成。这无法用几何光学解释，需要波动模型。
4. Features of the Single-Slit Diffraction Pattern | 单缝衍射图样的特点
The central maximum is the brightest and widest part of the pattern. Its width is roughly twice that of the secondary maxima. The intensity of the secondary maxima decreases rapidly away from the centre.
中央极大是整个图样中最亮、最宽的部分，其宽度约为次级极大的两倍。次级极大的强度从中心向外迅速减弱。
Dark fringes (minima) occur at angles θ where sinθ = mλ / w, with m = ±1, ±2, ±3… Here w is the slit width and λ is the wavelength. The central maximum lies between the first-order minima (m = ±1).
暗纹（极小）出现在满足 sinθ = mλ / w 的角度，其中 m = ±1, ±2, ±3…，w 为缝宽，λ 为波长。中央极大位于一级极小（m = ±1）之间。
5. Diffraction with White Light | 白光的衍射
When white light is used instead of a monochromatic source, the single-slit pattern shows a central white fringe, but the side fringes display spectra with violet on the inner side and red on the outer side of each order.
若使用白光代替单色光源，单缝图样显示中央条纹为白色，但两侧的条纹呈现出光谱，每一级的内侧为紫色，外侧为红色。
This occurs because longer wavelengths (red) diffract more and are deviated to larger angles, while shorter wavelengths (violet) diffract less. The overlapping of colours produces the rainbow-like pattern.
这是因为较长波长（红色）衍射更多，偏转角度更大；较短波长（紫色）衍射较少。各色光重叠产生了彩虹般的图样。
6. Diffraction as Evidence for the Wave Nature of Light | 衍射作为光波动性的证据
Historically, Newton’s corpuscular theory could not account for diffraction. The fact that light bends around obstacles and produces interference-like fringes strongly supports the wave model proposed by Huygens.
历史上，牛顿的微粒说无法解释衍射。光绕过障碍物并产生类似干涉的条纹这一事实，强有力地支持了惠更斯提出的波动模型。
In the modern understanding, both diffraction and interference are characteristic wave phenomena. The AQA IGCSE syllabus emphasises diffraction as a key piece of evidence that light behaves as a wave.
在现代认识中，衍射和干涉都是典型的波动现象。AQA IGCSE 课程强调衍射是证明光具有波动性的关键证据之一。
7. Factors Affecting the Amount of Diffraction | 影响衍射程度的因素
Two main factors determine how much a light wave spreads: the wavelength λ of the light and the size of the aperture w. Increasing λ or decreasing w increases the amount of diffraction.
决定光波扩散程度的主要因素有两个：光的波长 λ 和孔径大小 w。增大 λ 或减小 w 都会增加衍射量。
For a given slit, red light (longer λ) diffracts more than blue light (shorter λ). For a given wavelength, a narrower slit produces more spreading. These relationships are often tested in exam contexts.
对于给定的狭缝，红光（较长 λ）比蓝光（较短 λ）衍射更多。对于给定波长，更窄的狭缝产生更多扩散。这些关系常被考查。
8. Comparing Diffraction of Light, Sound and Water Waves | 光波、声波和水波衍射的比较
Diffraction is a universal wave property. Sound waves, with wavelengths of the order of metres, diffract easily around doorways and large obstacles, which is why we can hear around corners.
衍射是所有波的普遍性质。声波波长在米量级，容易绕过门口和大型障碍物发生衍射，因此我们能在拐角处听到声音。
Water waves diffract noticeably when harbour openings are comparable to their wavelength. For light, the very short wavelength means special narrow slits are needed to demonstrate diffraction clearly.
水波在港口开口宽度与其波长可比拟时，衍射明显。光由于波长非常短，需要特殊的窄缝才能清晰展示衍射。
This comparison helps students appreciate why light diffraction is less obvious in everyday life, yet it is still a fundamental wave behaviour.
这种比较有助于学生理解为什么光的衍射在日常生活中不那么显而易见，但它仍是一种基本的波动行为。
9. Common Exam Questions and Pitfalls | 常见考题与易错点
Examiners often ask students to describe the effect of changing slit width or wavelength on the diffraction pattern. A narrower slit gives a wider central maximum, not narrower – a common misconception.
考官常要求学生描述改变缝宽或波长对衍射图样的影响。更窄的狭缝产生更宽的中央亮纹，而非更窄——这是一个常见误解。
Another typical question is to sketch the pattern for red light and then for blue light, showing that the blue pattern has a narrower central maximum due to its shorter wavelength.
另一个典型问题是先画出红光的图样，再画出蓝光的，展示蓝光图样因波长较短而中央极大更窄。
Students should also be ready to explain why a diffraction pattern is not seen when a torch shines through a wide doorway – the gap (≈ 1 m) is enormously larger than the wavelength of light (≈ 5 × 10⁻⁷ m).
学生还应能解释为什么手电筒通过宽门口时看不见衍射图样——门宽（约 1 m）远大于光的波长（约 5 × 10⁻⁷ m）。
10. Summary and Revision Tips | 小结与复习建议
Recap: Light diffraction is the bending of light waves around obstacles or through slits, giving a characteristic pattern of bright and dark fringes due to constructive and destructive interference of wavelets from the slit.
回顾：光的衍射是光波绕过障碍物或穿过狭缝时弯曲的现象，由于狭缝处子波的相长和相消干涉，产生特征的明暗条纹图样。
For exam success, draw clear annotated diagrams of the single-slit pattern, learn the conditions for noticeable diffraction, and practise linking changes in slit width or wavelength to the observed pattern. Remember: diffraction = wave behaviour.
为了考试成功，画出清晰的单缝图样标注图，记住明显衍射的条件，并练习将狭缝宽度或波长的变化与观察到图样联系起来。记住：衍射 = 波动行为。
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