A-Level AQA Physics: Diffraction of Light | A-Level AQA 物理：光的衍射 考点精讲

📚 A-Level AQA Physics: Diffraction of Light | A-Level AQA 物理：光的衍射 考点精讲

Diffraction is a key wave phenomenon that provides evidence for the wave nature of light. In AQA A-Level Physics, understanding how light spreads after passing through apertures or around obstacles and how diffraction gratings produce interference patterns is essential. This article covers the core principles, equations, practical applications, and typical exam questions to help you master diffraction of light.

衍射是一种关键的波动现象，为光的波动性提供了证据。在 AQA A-Level 物理学中，理解光通过孔径或绕过障碍物后的扩展方式，以及衍射光栅如何产生干涉图样至关重要。本文涵盖核心原理、方程、实际应用和典型考题，帮助你掌握光的衍射。

1. What is Diffraction? | 什么是衍射？

Diffraction occurs when waves encounter an obstacle or a slit. The waves bend around the corners and spread out. For significant diffraction, the size of the gap should be comparable to the wavelength of the wave. If the gap is much larger than the wavelength, diffraction is minimal and the light travels in straight lines.

当波遇到障碍物或狭缝时会发生衍射。波在边角处弯曲并扩展。要发生明显的衍射，缝隙的尺寸应与波长相近。如果缝隙远大于波长，衍射极小，光沿直线传播。

Diffraction is a property of all waves, including sound, water, and light. It demonstrates that light behaves as a wave, as predicted by Huygens’ principle.

衍射是所有波（包括声波、水波和光波）的性质。它证明了光具有波动性，正如惠更斯原理所预测。

2. Single-Slit Diffraction | 单缝衍射

When coherent monochromatic light passes through a narrow single slit of width a, a pattern of bright and dark fringes appears on a distant screen. The central bright fringe is the widest and brightest. On either side, there are subsidiary maxima that are much dimmer and narrower.

当相干单色光通过宽度为 a 的窄单缝时，会在远处的屏幕上出现明暗相间的条纹。中央亮条纹最宽最亮。两侧有次级亮条纹，它们更加暗淡和狭窄。

The condition for dark fringes (destructive interference) is a sin θ = mλ, where m = 1, 2, 3,… and θ is the angle measured from the centre of the pattern.

暗条纹（相消干涉）的条件为 a sin θ = mλ，其中 m = 1, 2, 3,…，θ 是从图样中心测量的角度。

a sin θ = mλ

The first minimum (m = 1) gives the angular half-width of the central maximum: sin θ = λ / a. For small angles, θ ≈ λ / a, so the total angular width of the central bright fringe is Δθ ≈ 2λ / a.

第一极小（m = 1）给出中央亮纹的半角宽度：sin θ = λ / a。对于小角度，θ ≈ λ / a，因此中央亮纹的总角宽度为 Δθ ≈ 2λ / a。

3. Single-Slit Intensity Pattern | 单缝衍射强度图样

The intensity distribution of the single-slit pattern is not uniform. The central maximum contains most of the light energy. The intensity of the secondary maxima decreases rapidly as the order m increases. This is because more light is cancelled at larger angles.

单缝衍射图样的强度分布并不均匀。中央极大包含大部分光能。随着级数 m 增加，次级极大的强度迅速降低。这是因为在较大角度处有更多的光被抵消。

The width of the central maximum is inversely proportional to the slit width a; making the slit narrower widens the pattern.

中央极大的宽度与缝宽 a 成反比；缝隙越窄，图样越宽。

Δθ ≈ 2λ / a

If a is reduced, θ increases, so diffraction is more pronounced. In addition, a longer wavelength also increases the spreading.

如果 a 减小，θ 增大，衍射更明显。此外，波长越长，扩展越宽。

4. The Diffraction Grating | 衍射光栅

A diffraction grating consists of a large number of parallel, equally spaced slits. Commonly, it is made by ruling fine lines on a glass or plastic surface. The distance between adjacent slits is called the grating spacing d.

衍射光栅由大量平行、等间距的狭缝组成。通常是在玻璃或塑料表面刻划细线制成。相邻狭缝之间的距离称为光栅常数 d。

When light is incident on the grating, each slit acts as a source of diffracted wavelets that interfere. This produces a pattern of very sharp, bright maxima at specific angles, separated by wide dark regions.

当光照射光栅时，每个狭缝都作为衍射子波的波源并相互干涉。这会在特定角度产生非常尖锐、明亮的极大，中间有

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