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

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

Diffraction is one of the most fascinating behaviours of waves, and in GCSE Physics, understanding how light bends around obstacles and spreads through narrow gaps is essential. This revision guide covers every key concept, from single-slit patterns to real-world applications, helping you master the topic for your exam.

衍射是波动最迷人的行为之一。在 GCSE 物理中，理解光如何绕过障碍物以及穿过窄缝时如何扩展是至关重要的。这篇考点精讲涵盖了从单缝图样到实际应用的每一个关键概念，助你掌握该主题，轻松应考。

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. Light, being a transverse electromagnetic wave, exhibits diffraction in the same way as water waves or sound waves. The extent of spreading depends on the size of the gap compared with the wavelength of the light.

衍射是波在穿过缝隙或绕过障碍物时发生的扩展现象。光是一种横电磁波，它与水波或声波一样会发生衍射。扩展的程度取决于缝隙大小与光波长之间的比较关系。

In everyday life, you can observe diffraction when you hear sound from around a corner — sound diffracts because its wavelength is similar to the size of the door opening. Light has a much smaller wavelength, so its diffraction is harder to observe without specialised equipment.

日常生活中，你能听到从墙角传来的声音，就是因为声波发生了衍射——声音的波长与门洞大小相近。光的波长要小得多，所以没有专门设备时，光的衍射很难被观察到。

2. Conditions for Diffraction | 衍射发生的条件

For noticeable diffraction to occur, the size of the gap or obstacle must be approximately equal to the wavelength of the wave. When the gap is much larger than the wavelength, the wave passes through with very little spreading, producing sharp shadows in the case of light.

要发生明显的衍射，缝隙或障碍物的尺寸必须与波的波长大致相当。当缝隙远大于波长时，波几乎不发生扩展地穿过去，就光而言，会产生清晰的阴影。

If the gap width is around the same size as the light’s wavelength (about 4 × 10⁻⁷ m to 7 × 10⁻⁷ m for visible light), the emerging wavefront becomes almost semicircular, and the light spreads significantly. This is the fundamental condition for observing light diffraction patterns in the laboratory.

如果缝隙宽度与光的波长（可见光约为 4 × 10⁻⁷ m 到 7 × 10⁻⁷ m）相近，出射波前几乎呈半圆形，光显著扩展。这是在实验室中观察到光衍射图样的基本条件。

Comparison rule: gap width ≈ wavelength → maximum diffraction

比较规则：缝隙宽度 ≈ 波长 → 最大衍射

3. Diffraction of Light Through a Single Slit | 光通过单缝的衍射

When a parallel beam of monochromatic light (light of a single wavelength) passes through a narrow single slit, the light diffracts and forms a characteristic pattern on a screen placed behind it. The pattern consists of a bright central maximum, which is wider and more intense than the secondary maxima on either side, separated by dark minima.

当一束平行的单色光（单一波长的光）穿过一条狭窄的单缝时，光发生衍射，在后面的屏幕上形成特征图样。该图样包含一个亮中央主极大，它比两侧的次级极大更宽、更亮，并被暗纹分隔开。

The central bright fringe is twice as wide as the other bright fringes. This happens because the wavelets from all points across the slit interfere constructively at the centre, producing maximum illumination. The dark fringes occur where destructive interference takes place.

中央亮条纹的宽度是其他亮条纹的两倍。这是因为穿过缝隙各点的子波在中心处产生相长干涉，形成最大亮度。暗纹出现在相消干涉的位置。

In GCSE, you are not required to derive the equation, but you might recall that the condition for the first dark fringe is given by: a sin θ = λ, where ‘a’ is the slit width and θ is the angle. This formula shows that a smaller slit results in a wider central maximum.

在 GCSE 中，不要求推导公式，但你可能记得第一条暗纹的条件是：a sin θ = λ，其中 ‘a’ 是缝宽，θ 是角度。这个公式表明，缝越窄，中央主极大越宽。

4. The Diffraction Pattern: Bright and Dark Fringes | 衍射图样：明暗条纹

The diffraction pattern produced by a single slit is a direct consequence of interference between secondary wavelets. Huygens’ principle states that every point on a wavefront acts as a source of secondary wavelets; after passing through the slit, these wavelets spread out and overlap, creating regions of constructive and destructive interference.

单缝产生的衍射图样是二级子波之间干涉的直接结果。惠更斯原理表明波前上的每一点都可以视为次级子波的波源；穿过狭缝后，这些子波扩展并重叠，形成相长干涉和相消干涉区域。

Key features of the pattern:

图样的关键特征：

Central maximum: Bright, wide fringe directly opposite the slit. | 中央主极大：正对狭缝的亮、宽条纹。
Secondary maxima: Dimmer fringes on either side, of equal width. | 次级极大：两侧较暗的条纹，宽度相等。
Minima: Perfectly dark bands where waves cancel out. | 极小（暗纹）：波相消形成的完全暗带。

The intensity of the maxima decreases rapidly as you move away from the centre. In monochromatic light, the bright fringes all have the same colour; with white light, a colourful spectrum appears (see next section).

随着远离中心，极大处的强度迅速减小。在单色光下，亮条纹均为同一颜色；在白光下则会出现彩色光谱（见下一节）。

5. White Light Diffraction and Dispersion | 白光的衍射与色散

When white light is diffracted through a single slit or a diffraction grating, each wavelength within the white light is diffracted by a different amount. Since red light has a longer wavelength (≈ 700 nm) than blue light (≈ 400 nm), red light spreads out more, producing a continuous spectrum on the screen.

当白光通过单缝或衍射光栅发生衍射时，白光中的每种波长会以不同的角度衍射。由于红光的波长（≈ 700 nm）比蓝光（≈ 400 nm）长，红光扩展得更多，在屏幕上形成连续光谱。

The central maximum remains white because all wavelengths overlap constructively at the centre. However, on either side of the central white fringe, you observe spectra with violet on the inner side and red on the outer side. This separation of colours by diffraction is similar to, but should not be confused with, dispersion by a prism, although the underlying principle of wavelength-dependent bending is different.

中央主极大依然是白色，因为所有波长在中心处相长重叠。然而，在中央白色条纹的两侧，你可以观察到内紫外红的光谱。这种通过衍射实现的色彩分离与棱镜的色散相似但不应混淆，尽管波长导致偏转的原理不同。

For the exam, remember: the wider diffraction of red light means it appears further from the centre than blue light in the first-order spectrum.

考试中请记住：红光衍射更宽，意味着在第一级光谱中它比蓝光更远离中心。

6. Diffraction Grating (Higher Tier Only) | 衍射光栅（仅提高层级）

A diffraction grating consists of many equally spaced parallel slits (often hundreds or thousands per millimetre). When monochromatic light is incident on a grating, the diffracted beams from adjacent slits interfere, producing very sharp and bright maxima at specific angles.

衍射光栅由许多等间距的平行狭缝组成（通常每毫米有几百或几千条）。当单色光照射光栅时，来自相邻狭缝的衍射光束发生干涉，在特定角度产生非常尖锐而明亮的极大。

The grating equation is: d sin θ = nλ, where d is the spacing between slits, n is the order number (0, 1, 2, …), and λ is the wavelength. This equation allows you to calculate wavelength from the angle of a bright spot, making gratings useful in spectroscopy.

光栅方程为：d sin θ = nλ，其中 d 是狭缝间距，n 是级数（0, 1, 2, …），λ 是波长。该方程允许从亮点的角度计算波长，使光栅在光谱学中非常有用。

In comparison with a double slit, a diffraction grating produces maxima that are much sharper and brighter, which makes measurements of wavelength more precise. For GCSE, you may only need to describe the pattern and know that the grating produces spectra similar to single-slit diffraction but with higher resolution.

与双缝相比，衍射光栅产生的极大要锐利和明亮得多，这使得波长的测量更加精确。对于 GCSE，你可能只需描述图样并知道光栅产生的光谱与单缝衍射类似，但分辨率更高。

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

Three main factors govern how much a light wave diffracts: the wavelength of the light, the size of the opening or obstacle, and the distance from the source to the obstacle. The relationship is inverse between gap width and amount of spreading — smaller gap, more diffraction.

三个主要因素决定光波衍射的程度：光的波长、开口或障碍物的大小、以及波源到障碍物的距离。缝隙宽度与扩展量呈反比关系——缝隙越小，衍射越多。

Wavelength: Longer wavelengths diffract more than shorter ones. That’s why red light spreads more than violet. | 波长：长波衍射比短波多。所以红光比紫光扩展得多。
Gap size: If the gap is much larger than λ, diffraction is negligible; if comparable, diffraction is significant. | 缝隙大小：如果缝隙远大于 λ，衍射可忽略；如果相当，衍射显著。
Distance: The observed spreading increases with the distance from the slit to the screen. | 距离：观察到的扩展随缝隙到屏幕的距离增加而增加。

In exam questions, you may be asked to predict how the diffraction pattern changes when, for example, a narrower slit is used or the colour of light is changed from green to red. Always link your answer to the concepts of wavefront curvature and interference.

在考试题目中，你可能被要求预测当使用更窄的狭缝或将光的颜色从绿色变为红色时，衍射图样如何变化。回答时一定要联系波前弯曲和干涉的概念。

8. Comparing Diffraction of Light and Sound | 比较光与声音的衍射

Both light and sound are waves, but their wavelengths differ enormously. Sound waves in air have wavelengths from about 17 mm to 17 m (audible range), which are similar to the size of everyday objects like doors and windows. As a result, sound diffracts noticeably around corners — this is why you can hear someone speaking in an adjacent room even though you cannot see them.

光和声音都是波，但它们的波长差异巨大。声波在空气中的波长从约 17 mm 到 17 m（可听范围），与门、窗等日常物体的大小相似。因此，声音在墙角处明显衍射——这就是为什么你能听到隔壁房间的人说话而看不到他们。

Light, on the other hand, has a wavelength around 5 × 10⁻⁷ m, which is much smaller than typical openings. Thus, light travels in almost straight lines and casts sharp shadows, with minimal diffraction in daily scenarios. Only with very narrow slits, fine hairs, or diffraction gratings can we observe its diffraction.

另一方面，光的波长约为 5 × 10⁻⁷ m，远小于典型的开口尺寸。因此，光几乎沿直线传播，投射清晰阴影，在日常情境中衍射极少。只有使用极窄的狭缝、细丝或衍射光栅，我们才能观察到其衍射。

This comparison is a classic GCSE exam question: explain why sound diffracts through a doorway but light does not. The key is the relative size of wavelength to the opening.

这种比较是 GCSE 考试中的经典问题：解释为什么声音可以绕过门口衍射而光却不能。关键在于波长相对于开口的大小。

9. Practical Demonstration: Observing Light Diffraction | 实验演示：观察光衍射

In the school laboratory, the simplest way to demonstrate light diffraction is to shine a laser pointer through a single slit and project the pattern onto a distant wall or screen. A laser provides coherent, monochromatic light, which makes the pattern sharp and clear.

在学校实验室里，演示光衍射最简单的方法是用激光笔照射一个单缝，并把图样投射到远处的墙壁或屏幕上。激光提供相干、单色的光，使图样锐利清晰。

Typical setup:

典型装置：

Place a single slit (width ≈ 0.1 mm) in front of a laser. | 在激光前放置一个单缝（宽度约 0.1 mm）。
Observe the pattern on a screen at least 2 m away. | 在至少 2 m 外的屏幕上观察图样。
Measure the width of the central maximum and the distance to the screen; you can use similar triangles and the small-angle approximation (tanθ ≈ sinθ ≈ θ in radians) to estimate the wavelength, though this is more common at A-level. | 测量中央主极大的宽度和到屏幕的距离；你可以利用相似三角形和小角度近似（tanθ ≈ sinθ ≈ θ 以弧度计）估算波长，不过这更常见于 A-level。

Safety: Never look directly into the laser beam. Use a low-power Class 2 laser.

安全：切勿直视激光束，使用低功率的 2 类激光。

An alternative demonstration uses a human hair: when a laser beam is shone onto a single thin hair, the diffraction pattern produced is equivalent to that of a single slit of the same width. This is a memorable trick — a dark hair produces a bright central spot and symmetric fringes.

另一个演示是用人的头发：当激光照射到一根细发上时，产生的衍射图样等效于相同宽度的单缝衍射。这是一个令人印象深刻的技巧——一根暗色头发也能产生亮的中央斑点及对称条纹。

10. Common Misconceptions and Exam Tips | 常见误区与考试技巧

Misconception 1: ‘Diffraction only occurs with light.’ In fact, all waves diffract — sound, water, radio, etc. Remember that diffraction is a property of waves, not just light.

误区一：“衍射只发生在光中。”事实上，所有波都会衍射——声波、水波、无线电波等。请记住，衍射是波的性质，而不仅仅是光的。

Misconception 2: ‘A narrower slit makes the central maximum narrower.’ Actually, reducing the slit width increases the amount of diffraction, making the central maximum wider. Always check the relationship: narrower slit → wider pattern.

误区二：“更窄的狭缝使中央主极大变窄。”实际上，减小缝宽会增加衍射量，使中央主极大变宽。记得核对关系：更窄的狭缝 → 更宽的图样。

Misconception 3: ‘Diffraction and refraction are the same.’ They are completely different: diffraction involves spreading due to obstacles/gaps, while refraction involves bending due to a change in speed when entering a different medium.

误区三：“衍射和折射是同一回事。”它们完全不同：衍射是由于障碍物/缝隙引起的扩展，而折射是由于进入不同介质时速度改变引起的弯曲。

Exam tip: When describing the pattern, always use the terms ‘central maximum’, ‘fringes’, ‘monochromatic’, and ‘coherent’ correctly. If the question asks about safety, mention laser precautions.

考试技巧：描述图样时，要正确使用“中央主极大”、“条纹”、“单色”、“相干”等术语。如果问题涉及安全，要提到激光防护措施。

11. Summary and Key Equations | 总结与关键公式

Diffraction of light is evidence of its wave nature. The key points to remember for GCSE are:

光的衍射是光具有波动性的证据。GCSE 需要记住的要点有：

Diffraction: spreading of waves through a gap or around an obstacle. | 衍射：波通过缝隙或绕过障碍物时的扩展。
Condition: gap size ≈ wavelength for maximum effect. | 条件：缝隙大小 ≈ 波长，效果最显著。
Single-slit pattern: bright central maximum, twice the width of other maxima, with alternating dark fringes. | 单缝图样：亮中央主极大，宽度是其他极大的两倍，并有交替的暗纹。
White light diffraction: central white fringe, spectra on sides (violet inside, red outside). | 白光衍射：中央白色条纹，两侧为光谱（内紫外红）。
Grating equation (Higher): d sin θ = nλ. | 光栅方程（提高）：d sin θ = nλ。
Comparison: sound diffracts more because its wavelength is larger relative to everyday gaps. | 比较：声音衍射更多，因为其波长相较于日常缝隙更大。

d sin θ = nλ and a sin θ = nλ (for single slit minima, where n = 1, 2, …)

d sin θ = nλ 以及 a sin θ = nλ（针对单缝极小，其中 n = 1, 2, …）

Mastering this topic will not only help you score well in the waves section but also build a solid foundation for understanding interference and the electromagnetic spectrum. Good luck!

掌握这个专题不仅能帮助你在波的章节取得好成绩，还能为理解干涉和电磁波谱打下坚实基础。祝你好运！

Published by TutorHao | Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

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