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

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

Light diffraction is a key topic in the OCR GCSE Physics syllabus. Understanding how light spreads after passing through a gap or around an obstacle reveals its wave nature and links interference with practical applications such as spectroscopy and optical instruments. This article will cover the essential concepts, diagrams you need to interpret, and typical exam-style reasoning.

光的衍射是 OCR GCSE 物理大纲中的重要课题。理解光穿过缝隙或绕过障碍物后如何扩展，揭示了光的波动性，并将干涉与光谱学、光学仪器等实际应用联系起来。本文将涵盖核心概念、你需要解读的图样以及典型的考试推理方式。

1. What is Diffraction? | 什么是衍射？

Diffraction is the spreading out of a wave after it passes through a narrow gap or moves past an obstacle. It is a wave property exhibited by all types of waves, including sound, water ripples and light.

衍射是指波在穿过窄缝或绕过障碍物后发生扩展的现象。这是所有类型的波（包括声波、水波和光波）都表现出的波动性质。

For diffraction to be significant, the size of the gap or obstacle should be similar to the wavelength of the wave. If the gap is much larger than the wavelength, diffraction is barely noticeable.

要使衍射效果明显，缝隙或障碍物的尺寸应当与波的波长相近。如果缝隙比波长大得多，衍射几乎观察不到。

In everyday life, you hear sound around a corner because sound waves have wavelengths of a few metres, which easily diffract around doorways. Light has a much smaller wavelength (around 5 × 10⁻⁷ m), so its diffraction is only obvious with very narrow gaps.

在日常生活中，你能听到拐角处传来的声音，是因为声波的波长有几米，很容易绕射过门廊。光的波长非常小（约 5 × 10⁻⁷ m），因此只有在缝隙极窄的时候，其衍射才明显。

2. Single-Slit Diffraction of Light | 光的单缝衍射

When monochromatic light (light of a single colour) is directed at a thin slit, it diffracts and forms a pattern on a screen placed behind the slit. The pattern consists of a bright central fringe, flanked by alternating dark and bright fringes.

当单色光（单一颜色的光）照射到一个窄缝上时，光发生衍射，在后面的屏幕上形成图样。这个图样由一条明亮的中央条纹及其两侧交替出现的暗条纹和亮条纹组成。

The central maximum is the widest and brightest fringe. The first-order bright fringes on either side are much dimmer and narrower. The intensity decreases rapidly as you move away from the centre.

中央极大是最宽、最亮的条纹。两侧的一级亮条纹要暗得多，也窄得多。随着远离中心，光强度迅速减弱。

This pattern is different from the double-slit interference pattern, where bright fringes are equally spaced and have nearly the same intensity. The single-slit pattern is dominated by the broad central region.

这种图样与双缝干涉图样不同，后者的亮条纹是等间距的且强度几乎相同。单缝图样则由宽阔的中央区域主导。

3. Explaining the Diffraction Pattern | 解释衍射图样

Huygens’ principle helps to explain the pattern: every point on the wavefront inside the slit acts as a source of secondary wavelets. These wavelets spread out and combine, or interfere with each other, at the screen.

惠更斯原理有助于解释图样：狭缝内波前上的每一点都作为次级小波源。这些小波向外扩展，在屏幕上相遇并叠加（即发生干涉）。

Waves from different points across the slit travel slightly different distances to reach the same spot on the screen. When they arrive in phase, constructive interference produces a bright fringe. When they arrive out of phase, destructive interference produces a dark fringe.

从狭缝不同位置发出的波到达屏幕上同一点时，经过的路程略有不同。当它们同相到达时，发生相长干涉，形成亮条纹；反相到达时发生相消干涉，形成暗条纹。

The central bright fringe is formed by wavelets that travel equal (or nearly equal) path lengths, so they interfere constructively. The first dark spots on either side occur where the path difference between wavelets from the top and middle of the slit equals a half-wavelength (λ/2).

中央亮条纹由路程相等（或几乎相等）的小波叠加产生相长干涉形成。两侧的第一个暗点位置，对应狭缝顶部和中点发出的子波路程差为半个波长（λ/2）。

The condition for a dark fringe in a single slit is: a sin θ = n λ, where a is the slit width, θ is the angle to the dark fringe, n is a whole number (1, 2, 3…) and λ is the wavelength. The central maximum lies between the first dark fringes (n=1).

单缝暗条纹条件为：a sin θ = n λ，其中 a 是缝宽，θ 是暗条纹对应的角度，n 是整数（1, 2, 3…），λ 是波长。中央极大位于一级暗条纹（n=1）之间。

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

Two main factors determine how much the light spreads: the wavelength of the light and the slit width.

决定光扩散程度的主要因素有两个：光的波长和狭缝的宽度。

Increasing the wavelength (using red light instead of blue) makes the pattern wider. The central fringe becomes broader and bright fringes spread further apart.

增大波长（例如用红光代替蓝光）会使图样变宽。中央条纹变得更宽，亮条纹之间间距更大。
Decreasing the slit width a also increases the amount of spreading. A narrower slit causes more noticeable diffraction because the gap is closer to the wavelength scale.

减小缝宽 a 同样会增加衍射扩展。窄缝使衍射更明显，因为缝隙尺寸更接近波长尺度。

You can remember this relationship with the small-angle approximation: θ ≈ λ / a for the first minimum. When λ increases or a decreases, θ gets larger, indicating greater spreading.

你可以利用小角度近似公式记住这一关系：第一暗条纹处 θ ≈ λ / a。当 λ 增大或 a 减小时，θ 增大，表明扩散更厉害。

Using white light, you can directly see how wavelength affects the angle: red light diffracts more than blue, forming spectra on either side.

使用白光时，你能直接观察到波长如何影响偏转角度：红光比蓝光衍射得更多，在中央两侧形成光谱。

5. White Light Diffraction | 白光衍射

When a narrow slit is illuminated with white light, the diffraction pattern becomes a spectrum. The central maximum is white because all wavelengths overlap constructively at the centre.

用白光照射窄缝时，衍射图样变成光谱。中央极大呈白色，因为所有波长在中心位置相长叠加。

On each side of the central white fringe, spectra are formed. The inner edge of each first-order spectrum is violet, and the outer edge is red. This is because violet light (shorter wavelength) diffracts less and appears closer to the centre, while red light (longer wavelength) diffracts more and appears further out.

在中央白色条纹的每侧都形成光谱。每一条一级光谱的内侧是紫色，外侧是红色。这是因为紫光（波长较短）衍射较少，靠中心更近；红光（波长较长）衍射更多，距中心更远。

At GCSE, you are often asked to sketch the appearance of white light shining through a single slit: a white central line, coloured bands on both sides with violet nearest the centre, and decreasing intensity beyond.

在 GCSE 考试中，经常要求你画出白光通过单缝的样子：一条白色中央线，两侧有彩色条纹，紫色离中心最近，再往外强度减弱。

6. Diffraction Gratings | 衍射光栅

A diffraction grating is a slide with many equally spaced parallel slits – typically hundreds or thousands of lines per millimetre. When light passes through a grating, the waves from each slit diffract and then interfere to produce very sharp, bright maxima.

衍射光栅是一块带有许多等距平行狭缝的玻片，通常每毫米有数百甚至数千条刻线。光通过光栅时，每个狭缝都发生衍射，然后相互干涉，产生非常锐利、明亮的极大。

The grating equation is: d sin θ = n λ, where d is the distance between adjacent slits (the grating spacing), θ is the angle to the bright fringe, n is the order number (0, 1, 2…) and λ is the wavelength.

光栅方程为：d sin θ = n λ，其中 d 是相邻狭缝间距（光栅常数），θ 是亮条纹的衍射角，n 是级数（0, 1, 2…），λ 是波长。

The central bright fringe (n=0) is called the zero order. The first bright fringes on either side are the first order, and so on. The maxima are narrow and well separated, which makes diffraction gratings much more precise for measuring wavelengths than double slits.

中央亮纹（n=0）称为零级。两侧第一组亮纹为一级，以此类推。极大又细又分离得开，这使得衍射光栅比双缝更适合精确测量波长。

If the light is not monochromatic, each order produces its own spectrum, with red deviated more than violet. This property is used in spectrometers to analyse light from stars or to identify elements.

如果光不是单色的，每一级都会产生自己的光谱，红光偏转角度比紫光大。这一特性被用于光谱仪中，以分析来自恒星的光或鉴别元素。

7. Interference vs. Diffraction | 干涉与衍射

Students often confuse interference and diffraction. Diffraction refers to the bending of waves as they pass through a gap or around a barrier. Interference is the superposition of two or more waves arriving at a point, resulting in reinforcement or cancellation.

学生常混淆干涉和衍射。衍射指波通过缝隙或绕过障碍物时的弯曲。干涉则是两个或多个波到达某一点时的叠加，产生加强或抵消。

In the single-slit pattern, the dark and bright fringes are caused by interference of wavelets originating from the same slit – so diffraction and interference go together. In the double-slit experiment, the two slits act as coherent sources, and we observe an interference pattern of equally spaced fringes, but each slit also produces its own single-slit diffraction envelope.

在单缝图样中，明暗条纹是由来自同一条缝的子波发生干涉引起的——因此衍射和干涉是同时发生的。在双缝实验中，两条缝充当相干光源，我们观察到等间距的干涉条纹，但每条缝自身也会产生单缝衍射包络。

At GCSE, you are expected to describe the key differences: a single slit gives a wide central maximum and dimmer outer fringes, while a double slit gives a series of equally spaced bright fringes of similar intensity (if you ignore the single-slit envelope). A diffraction grating narrows the fringes and makes them much brighter, with large dark spaces between orders.

在 GCSE 阶段，你要能描述关键区别：单缝产生宽的中央极大和较暗的外侧条纹，双缝则产生一系列等间距、强度大致相同的亮条纹（不考虑单缝包络）。衍射光栅使条纹变窄且更亮，各级之间有较大的暗区。

Table comparing patterns:

图样对比表格：

Feature / 特征	Single slit / 单缝	Double slit / 双缝	Diffraction grating / 光栅
中央条纹宽度	宽	与其它条纹等宽	很窄
条纹强度分布	中央强，向两侧递减	大致相同	各级极强，暗区大
条纹间距	由中央向外逐渐变密	均匀	随级数增大间距略微变化，但明亮条纹明显分离

8. Real-World Applications and Exam Tips | 实际应用与考试技巧

Diffraction of light has many practical uses. CD and DVD discs store data in tiny pits arranged in spiral tracks; when you view a disc under white light, the tracks act like a reflection diffraction grating, producing the rainbow colours.

光的衍射有很多实际用途。CD 和 DVD 光盘以微小的坑排列在螺旋轨道上储存数据；在白光下观察碟片时，轨道相当于一个反射式衍射光栅，产生了彩虹般的色彩。

The resolution of microscopes and telescopes is limited by diffraction. Light passing through the instrument’s aperture diffracts, making two close objects appear as one if their diffraction patterns overlap too much.

显微镜和望远镜的分辨率受衍射限制。光通过仪器的孔径时发生衍射，如果两个靠得很近的物体的衍射图样重叠太多，它们看起来就像一个物体。

When tackling exam questions on diffraction:

处理衍射考题时：

Always mention that diffraction is a wave property. It proves the wave nature of light.

始终要提及衍射是波的特性，证明了光的波动性。
Use the correct equation from the formula sheet: d sin θ = n λ for gratings, and know that for a single slit the first dark fringe satisfies a sin θ = λ.

使用公式表上的正确方程：光栅用 d sin θ = n λ，并知道单缝第一暗条纹满足 a sin θ = λ。
Be ready to explain how changing wavelength or slit width affects the pattern. Red light produces wider spacing than blue.

准备好解释改变波长或缝宽如何影响图样。红光比蓝光产生更宽的间距。
If asked to compare experiments, refer to the table above and explain why each pattern looks different.

如果要求比较实验，参考上表并解释为何每种图样看起来不同。
When calculating, always convert units – spacing d is often given in mm or lines per mm. For example, 300 lines/mm means d = 1/300 mm = 3.33 × 10⁻³ mm = 3.33 × 10⁻⁶ m.

计算时务必换算单位——间距 d 常以 mm 或每毫米线数给出。例如，300 线/毫米意味着 d = 1/300 mm = 3.33 × 10⁻³ mm = 3.33 × 10⁻⁶ m。

9. Common Misconceptions | 常见误区

One common misunderstanding is that diffraction requires an obstacle exactly the size of the wavelength. In truth, diffraction occurs whenever a wave encounters a gap, but the effect is most pronounced when the gap size is comparable to the wavelength.

一个常见误解是衍射要求障碍物尺寸恰好等于波长。实际上，只要波遇到缝隙就会发生衍射，只是当缝隙尺寸与波长相近时效果最显著。

Another misconception is that the single-slit pattern is due to simple ‘bending’ of light with no interference. The fringes are truly an interference effect between wavelets from the same slit, and without interference you would only see a fuzzy edge.

另一个误区是认为单缝图样仅仅是由于光发生了”弯曲”，没有干涉。实际上条纹是来自同一狭缝的子波间的干涉结果，如果没有干涉，你只会看到模糊的边缘。

Students sometimes think that a wider slit produces more diffraction. The opposite is true: a larger slit width results in a narrower diffraction pattern because the spreading angle θ decreases.

学生有时以为更宽的缝会产生更多衍射。事实恰恰相反：缝宽越大，衍射图样越窄，因为扩展角 θ 减小了。

Finally, do not assume diffraction gratings work only by diffraction – they combine diffraction and interference from multiple slits. The bright maxima positions are given by the same condition as constructive interference.

最后，不要认为衍射光栅只依靠衍射——它们同时利用了多个狭缝的衍射和干涉。明亮极大的位置与相长干涉的条件一致。

10. Summary of Key Points | 考点总结

Diffraction is the spreading of waves through a gap or around an obstacle; it is significant when gap size ≈ wavelength.

衍射是波通过缝隙或绕过障碍物时的扩展现象；当缝隙尺寸与波长相近时效果显著。
Single-slit diffraction with monochromatic light gives a wide central bright fringe and fainter outer fringes.

单色光单缝衍射产生宽的中央亮条纹和较暗的外侧条纹。
Path difference between wavelets leads to constructive (bright) or destructive (dark) interference, described by a sin θ = nλ for dark fringes.

子波之间的路程差导致相长（亮）或相消（暗）干涉，暗条纹满足 a sin θ = nλ。
Spreading increases with longer wavelength and narrower slit: θ ≈ λ/a for small angles.

波长越长、缝越窄，扩散越厉害：小角度下 θ ≈ λ/a。
White light produces a central white fringe and spectra with violet on the inside, red on the outside.

白光产生中央白色条纹，两侧光谱内紫外红。
A diffraction grating produces sharp, well-separated maxima given by d sin θ = nλ; used for measuring wavelength and analysing light.

衍射光栅产生锐利分离的极大，由 d sin θ = nλ 决定；用于测量波长和分析光。
Diffraction and interference are linked: diffraction describes bending; interference describes superposition creating the pattern.

衍射和干涉相互关联：衍射描述波的弯曲；干涉描述叠加产生图样。
Common exam tasks: sketching patterns, comparing single/double slits and gratings, calculating wavelength or spacing, and explaining colour separation.

常见考题：画出图样、比较单缝/双缝和光栅、计算波长或间距、解释色散。

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