Diffraction of Light | 光的衍射 考点精讲

📚 Diffraction of Light | 光的衍射 考点精讲

Diffraction is a fundamental wave phenomenon that occurs when a wave encounters an obstacle or a slit. For IGCSE OCR Physics, understanding how light diffracts provides key evidence of light’s wave nature, and the conditions under which diffraction becomes noticeable are essential. This article covers every required aspect of the topic, from basic concepts to exam-style applications, with clear pairings of English and Chinese explanations.

衍射是波遇到障碍物或狭缝时发生的基本波动现象。对于IGCSE OCR物理而言，理解光如何衍射是为光的波动性提供关键证据，同时必须掌握让衍射变得明显的条件。本文覆盖该主题所有考点，从基本概念到考试题型的应用，均以清晰的中英对照方式呈现。

1. What is Diffraction? | 什么是衍射？

Diffraction is the spreading of waves as they pass through a gap or move around an obstacle. Instead of travelling in perfectly straight lines, waves bend into the geometrical shadow region. This behaviour is a property of all waves, including water waves, sound waves, and light waves. The amount of diffraction depends on the relative sizes of the wavelength and the gap or obstacle.

衍射是波通过缝隙或绕行障碍物时发生的扩展现象。波并不会完全沿直线传播，而是会弯曲进入几何阴影区。这种行为是所有波共有的特性，包括水波、声波和光波。衍射的程度取决于波长与缝隙或障碍物大小的相对关系。

2. Diffraction of Water Waves | 水波的衍射

Water waves are an excellent way to visualise diffraction. In a ripple tank, when straight waves pass through a wide opening, the wavefronts remain mostly straight and only the edges curve slightly. As the gap is narrowed to a size comparable to the wavelength, the waves fan out in nearly semicircular patterns. This demonstrates the key rule: significant diffraction occurs when the gap width is similar to or smaller than the wavelength.

水波是观察衍射的绝佳方式。在波纹槽中，当直线波通过一个宽开口时，波前基本保持直线，只有边缘轻微弯曲。当缝隙缩小到与波长相近的尺寸时，波会以近乎半圆的形式扩散开来。这展示了一条关键规则：当缝隙宽度接近或小于波长时，会发生明显的衍射。

3. Diffraction of Sound Waves | 声波的衍射

Sound waves have wavelengths ranging from a few centimetres to several metres, which are often comparable to everyday doorways and obstacles. This is why you can hear someone speaking in the next room even if you cannot see them—sound diffracts around corners. The long wavelength of low-frequency sounds allows them to spread widely, while short-wavelength, high-frequency sounds diffract less and are more directional.

声波的波长范围从几厘米到几米不等，常常与日常生活中的门口或障碍物尺寸相当。这就是为什么即使看不见隔壁房间的人，也能听到他们说话——声音绕过了角落发生衍射。低频声音的波长较长，能广泛扩散，而高频声音波长短，衍射较少，方向性更强。

4. Why Is Light Diffraction Hard to Observe? | 为什么光的衍射难以观察？

Light has an extremely small wavelength, around 4 × 10⁻⁷ m to 7 × 10⁻⁷ m for visible light. For noticeable diffraction, the gap or obstacle must be of a similar size. Everyday openings are millions of times larger than light’s wavelength, so light appears to travel in straight lines, creating sharp shadows. Only with a very narrow slit—comparable to the wavelength—does light spread out observably.

光的波长非常小，可见光大约在 4 × 10⁻⁷ 米到 7 × 10⁻⁷ 米之间。为了产生明显的衍射，缝隙或障碍物必须具有相近的尺寸。日常生活中的缝隙比光的波长大数百万倍，因此光看起来沿直线传播，形成清晰的影子。只有当缝隙极窄、与波长接近时，光才会明显地扩散开来。

5. Single-Slit Diffraction with Monochromatic Light | 单色光的单缝衍射

When a laser beam of monochromatic light is directed at a narrow single slit, a diffraction pattern appears on a screen placed beyond the slit. The pattern consists of a bright central maximum flanked by a series of dimmer, narrower bright fringes separated by dark bands. This happens because each point on the wavefront passing through the slit acts as a source of secondary wavelets, which interfere constructively and destructively.

当一束单色激光照射到一条狭窄的单缝上时，在缝后放置的屏幕上会出现衍射图样。图样包含一个明亮的中央亮纹，两侧分布着一系列较暗、较窄的明条纹，之间被暗带隔开。这是因为通过狭缝的波前上的每一点都充当了次级子波源，这些子波发生相长干涉和相消干涉。

Condition for the first minimum: a sin θ = λ

第一级暗纹条件：a sin θ = λ

Where ‘a’ is the slit width, ‘θ’ is the angle to the first dark fringe, and ‘λ’ is the wavelength. In IGCSE, you do not need to perform detailed calculations, but you should know that the central maximum is twice as wide as the secondary maxima.

其中 ‘a’ 是缝宽，’θ’ 是到第一暗纹的角，’λ’ 是波长。在IGCSE阶段，你不需要进行详细计算，但应当知道中央亮纹的宽度是次级亮纹宽度的两倍。

6. The Single-Slit Pattern: Features | 单缝衍射图样：特征

The central bright fringe is the widest and most intense. On either side, the intensity of the bright fringes decreases rapidly, and each successive bright fringe is narrower than the previous one. Dark fringes are perfectly destructive interference positions. The symmetry of the pattern about the centre confirms that the light is spreading uniformly from the slit.

中央明条纹是最宽、最亮的。在其两侧，明条纹的亮度迅速下降，且每条后续的明条纹都比上一条更窄。暗条纹是完全相消干涉的位置。图样关于中心对称，证实了光从狭缝处均匀扩散。

Key characteristics can be summarized as:

主要特征可总结如下：

– Central maximum: brightest and widest, typically occupying the angular range up to the first minima on both sides.

– 中央亮纹：最亮、最宽，通常占据从中心到两侧第一级暗纹的角度范围。

– Secondary maxima: much dimmer and narrower; their peak intensities are only a few percent of the central maximum.

– 次级亮纹：暗淡且更窄；峰值强度仅为中央亮纹的百分之几。

– Dark fringes: positions where waves from different parts of the slit cancel out completely.

– 暗条纹：来自狭缝不同部分的波完全抵消的位置。

– Width of central maximum increases with wavelength and decreases with slit width: w ∝ λ / a

– 中央亮纹宽度随波长增大而增加，随缝宽增大而减小：w ∝ λ / a

7. White Light Diffraction | 白光的衍射

When white light passes through a single slit, the diffraction pattern shows a white central maximum because all colours overlap at the centre. On either side, however, spectra appear with violet light diffracted the least and red light diffracted the most. This separation of colours occurs because different wavelengths are diffracted by different amounts—shorter wavelengths bend less, longer wavelengths bend more.

当白光通过单缝时，衍射图样的中央亮纹是白色的，因为所有颜色的光在中央重叠。然而，在两侧会出现光谱，紫光衍射最少，红光衍射最多。这种颜色分离的发生是由于不同波长的光衍射程度不同——短波长弯曲较小，长波长弯曲较大。

This is a useful demonstration to emphasise that diffraction is wavelength-dependent. However, the IGCCE OCR exam typically focuses on monochromatic laser light to make the pattern clearer.

这是一个很有用的演示，强调了衍射与波长有关。不过，IGCSE OCR 考试通常聚焦于单色激光，以使图样更清晰。

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

Students sometimes confuse diffraction with two-source interference (such as Young’s double-slit experiment). Diffraction is the spreading of a wave from a single aperture, and the subsequent pattern arises from the interference of wavelets originating from different points across that same aperture. In double-slit interference, the pattern is created by the superposition of waves from two distinct coherent sources. Diffraction is present in both, but double-slit produces evenly spaced fringes modulated by a single-slit envelope.

学生有时会混淆衍射与双源干涉（如杨氏双缝实验）。衍射是波从单一孔径扩散，其后的图样源于同一孔径上不同点发出的子波之间的干涉。在双缝干涉中，图样是由两个独立的相干源的波叠加而成。衍射其实在两者中都存在，但双缝产生的条纹间距均匀，并且会受单缝包络的调制。

For IGCSE OCR, you are expected to recognise that a single slit produces a diffraction pattern with a prominent central band and decreasing side bands, while a double slit gives a series of bright and dark fringes of almost equal spacing and intensity (if slit widths are narrow enough).

对于IGCSE OCR，你需要认识到单缝产生的是一个有突出中央带和递减侧带的衍射图样，而双缝给出的是一系列明暗条纹，间距和强度几乎相等（若狭缝足够窄）。

9. Factors Affecting Diffraction | 影响衍射的因素

The two principal factors that determine the amount of diffraction are the wavelength of the wave and the size of the gap or obstacle. When the wavelength is much smaller than the gap, diffraction is negligible and the wave travels in straight lines. When the wavelength is comparable to or larger than the gap, the wave spreads significantly. For light, using a very narrow slit (of the order of a few micrometres) makes diffraction observable.

决定衍射程度的两个主要因素是波长以及缝隙或障碍物的尺寸。当波长远小于缝隙时，衍射可以忽略，波沿直线传播。当波长与缝隙相当或更大时，波会显著扩展。对于光来说，需要使用极窄的狭缝（量级为几微米）才能使衍射变得可见。

Moreover, the distance from the slit to the screen also affects the observed pattern size: a longer distance gives a wider spread of the pattern, but the angular positions of minima remain constant for a given slit and wavelength.

此外，从缝到屏幕的距离也会影响观察到的图样大小：距离越大，图样展开得越宽，但给定狭缝和波长下暗纹的角位置保持不变。

10. Diffraction as Evidence for the Wave Nature of Light | 衍射作为光波动性的证据

Historically, the observation that light can spread around obstacles and produce interference-like patterns supported the wave theory of light over the particle theory. If light were composed of particles, it would cast perfectly sharp shadows and would not bend. The single-slit experiment, in particular, shows that light undergoes diffraction and interference—behaviours characteristic of waves. This evidence, alongside polarisation and two-source interference, firmly establishes light as a transverse wave.

历史上，观察到光可以绕过障碍物并产生类似干涉的图样，支持了光的波动理论，而非粒子理论。如果光由粒子构成，它会投出边缘极为清晰的影子，而不会弯曲。特别是单缝实验表明，光会发生衍射和干涉——这是波的特征行为。这一证据，连同偏振和双源干涉，牢固地确立了光是一种横波。

In the IGCSE syllabus, you may be asked to explain how diffraction provides evidence that light behaves as a wave, so linking this phenomenon to the broader wave model is important.

在IGCSE大纲中，你可能会被要求解释衍射如何提供光具有波动行为的证据，因此将这一现象与更广泛的波动模型相联系非常重要。

11. Common Exam Questions and Tips | 常见考题与技巧

Typical IGCSE OCR questions on diffraction ask you to describe an experiment to demonstrate the diffraction of light, to sketch and label a single-slit diffraction pattern, or to explain why sound diffracts more noticeably than light in everyday situations. You may also need to compare single-slit and double-slit patterns. Some questions require you to state how the pattern changes if the slit width is decreased or if the wavelength is increased.

典型的IGCSE OCR衍射考题会要求你描述一个演示光衍射的实验，画出并标注单缝衍射图样，或者解释为何声音在日常情况下衍射比光更明显。你可能还需要比较单缝和双缝图样。有些题目要求陈述如果狭缝变窄或波长增大，图样会如何变化。

Key exam tips: Always label the central maximum as the brightest and widest band. When drawing patterns, ensure the central fringe is about twice the width of the first side fringe. For explanations, consistently link the outcome to the relationship between wavelength and gap size. Use terms like ‘constructive interference’ and ‘destructive interference’ accurately. For white light, mention that red is diffracted the most and violet the least.

关键考试技巧：始终将中央亮纹标记为最亮、最宽的带。在画图样时，确保中央明纹宽度约为第一侧明纹宽度的两倍。在解释时，始终将结果与波长和缝隙大小的关系联系起来。准确使用“相长干涉”和“相消干涉”等术语。对于白光，要提到红光衍射最多，紫光最少。

Remember that a small slit width is essential to observe light diffraction. In the lab, a laser and a thin slit (or a pair of razor blades) are commonly used. Finally, practise interpreting graphs of intensity versus position, as these can appear on the exam paper.

记住，很小的缝宽是观察到光衍射的关键。在实验中，通常使用激光和薄缝（或一对剃须刀片）。最后，练习解读强度随位置变化的图形，因为它们可能出现在试卷中。

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Published by TutorHao | Physics Revision Series | aleveler.com

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