📚 Evidence in A-Level Physics: From Rutherford to Quantum Duality | A-Level物理中的证据：从卢瑟福到量子二象性
In AQA A-level Physics, 'evidence' is not a side note: it is the bridge between observation and theory. Many exam questions ask you to describe how a particular experiment provides evidence for a model, interpret a graph, or use data to test a claim.
在AQA A-level物理中，“证据”不是边角知识：它是连接观察与理论的桥梁。许多考题要求你描述某个实验如何为模型提供证据、解释图线，或使用数据检验观点。
1. What Counts as Evidence in Physics? | 物理学中什么算证据？
Evidence means reproducible observations or measurements that support or contradict a model. A single result may be an anomaly, but repeated, controlled experiments build confidence in a theory.
证据指可重复的观察或测量，支持或反驳某个模型。单一结果可能是异常，但重复、受控的实验会增强对理论的信心。
In AQA papers, you should always link the evidence to the specific feature of the model: for example, large-angle scattering implies a concentrated positive charge.
在AQA试卷中，你应始终将证据与模型的具体特征联系起来：例如，大角度散射意味着正电荷集中。
2. Evidence for the Nuclear Atom: Rutherford's Gold Foil Experiment | 核式原子的证据：卢瑟福金箔实验
Geiger and Marsden, under Rutherford, fired alpha particles at thin gold foil. Most particles passed straight through, a few were deflected by small angles, and roughly 1 in 8000 bounced back at angles greater than 90°.
盖革和马斯登在卢瑟福指导下，用α粒子轰击薄金箔。大多数粒子径直穿过，少数发生小角度偏转，大约每8000个中有一个以大于90°的角度反弹回来。
The straight-through particles showed that the atom is mostly empty space, while the large-angle scattering showed that the positive charge and most of the mass are concentrated in a tiny nucleus.
直线穿过的粒子表明原子大部分是空的，而大角度散射表明正电荷和绝大部分质量集中在一个极小的原子核中。
r ≈ 10⁻¹⁴ m (nucleus) vs r ≈ 10⁻¹⁰ m (atom)
3. Interpreting Scattering Data | 散射数据的解释
The closer an alpha particle approaches the nucleus, the greater the electrostatic repulsion and the larger the deflection. By energy conservation, the distance of closest approach gives an upper limit for the nuclear radius:
α粒子越靠近原子核，静电排斥力越大，偏转越大。根据能量守恒，最接近距离给出了核半径的上限：
Eₖ = Q q / (4 π ε₀ d)
Here Q is the nuclear charge, q is the alpha particle charge, and d is the closest approach. This estimate is about 10⁻¹⁴ m, confirming the nucleus is about 10,000 times smaller than the atom.
这里Q是核电荷，q是α粒子电荷，d是最接近距离。该估计约为10⁻¹⁴ m，证实原子核比原子小约一万倍。
4. Conservation Laws as Evidence in Particle Interactions | 粒子相互作用中守恒定律作为证据
When particles decay or interact, quantities such as charge, baryon number, lepton number, and strangeness are conserved. Observing conservation can identify unknown particles or confirm a reaction.
当粒子衰变或相互作用时，电荷、重子数、轻子数和奇异数等量守恒。观察守恒关系可以识别未知粒子或确认反应。
- In beta-minus decay, a neutron changes to a proton, so charge is conserved but energy and momentum seemed not to be - evidence for the neutrino.
- 在β⁻衰变中，中子变为质子，电荷守恒但能量和动量似乎不守恒——这就是中微子存在的证据。
- In strange particle production, strangeness is conserved in strong interactions but not in weak decays, explaining the long lifetime of kaons.
- 在奇异粒子产生中，强相互作用下奇异数守恒，而弱衰变中不守恒，这解释了K介子的长寿命。
This section is frequently examined through tables of quark content and decays.
这一部分常通过夸克组成和衰变表格来考查。
5. Evidence for the Particle Nature of Light: The Photoelectric Effect | 光的粒子性的证据：光电效应
Ultraviolet light shining on a metal surface can eject electrons. The key observations are: emission is instantaneous, there is a threshold frequency below which no electrons are emitted, and increasing intensity only increases the number of emitted electrons, not their maximum kinetic energy.
紫外线照射金属表面可以打出电子。关键观察是：发射是瞬时的，存在一个截止频率，低于该频率不会发射电子；增加光强只增加发射电子数，不增加其最大动能。
tensity. Einstein explained them by proposing light consists of photons of energy E = h f.
这些观察无法用波动模型解释，因为波动模型中能量应逐渐积累并取决于强度。爱因斯坦提出光由光子组成，光子能量E = h f，从而解释了这些现象。
E = h f
6. Key Graphs and Threshold Frequency | 关键图线与截止频率
The photoelectric equation relates photon energy, work function, and maximum kinetic energy:
光电效应方程将光子能量、逸出功和最大动能联系起来：
Eₖ(max) = h f − φ
ximum kinetic energy against frequency gives a straight line with gradient equal to Planck's constant h and x-intercept equal to the threshold frequency f₀ = φ / h.
将最大动能对频率作图，得到一条直线，其斜率等于普朗克常数h，横截距等于截止频率 f₀ = φ / h。
Below f₀, no electrons are emitted no matter how intense the light is. This graph is direct evidence for energy quantisation.
低于 f₀ 时，无论光强多大都没有电子发射。该图线是能量量子化的直接证据。
7. Evidence for the Wave Nature of Electrons: Electron Diffraction | 电子波动性的证据：电子衍射
Electrons accelerated through a potential difference V gain kinetic energy e V. Their de Broglie wavelength is given by:
电子经电势差V加速后获得动能 e V。其德布罗意波长为：
λ = h / p = h / √2 m e V
When a beam of electrons is passed through graphite or a thin metal crystal, a diffraction pattern of concentric rings appears on a screen. Diffraction is a property of waves, so this is evidence that electrons have wave-like behaviour.
当电子束穿过石墨或薄金属晶体时，屏幕上出现同心环衍射图样。衍射是波动性质，因此这是电子具有波动行为的证据。
The ring spacing changes with accelerating voltage: higher voltage gives shorter wavelength, which gives smaller ring spacing, exactly as predicted by de Broglie's relation.
环间距随加速电压变化：电压越高，波长越短，环间距越小，与德布罗意关系预测完全一致。
8. De Broglie Wavelength and Diffraction Conditions | 德布罗意波长与衍射条件
r an electron accelerated through about 100 V, λ ≈ 1.2 × 10⁻¹⁰ m, matching atomic spacing.
只有当波长与原子间距（通常约10⁻¹⁰ m）相当时，衍射才明显。电子经约100 V加速后，λ ≈ 1.2 × 10⁻¹⁰ m，与原子间距匹配。
This explains why we do not observe diffraction of everyday objects: their de Broglie wavelength is far too small.
这解释了为什么我们观察不到日常物体的衍射：它们的德布罗意波长太小了。
9. Comparing Evidence: Wave-Particle Duality | 证据比较：波粒二象性
The photoelectric effect provides evidence that light can behave as a particle, while electron diffraction provides evidence that electrons can behave as waves. Together they show that both light and matter have a dual nature.
光电效应为光的粒子性提供证据，而电子衍射为电子的波动性提供证据。两者共同表明光和物质都具有波粒二象性。
In AQA exam answers, avoid saying 'light is a particle' or 'electrons are waves'. Instead say 'light shows particle-like behaviour in the photoelectric effect' and 'electrons show wave-like behaviour in diffraction'.
在AQA考试答案中，避免说“光是粒子”或“电子是波”。而要说“光在光电效应中表现出粒子行为”和“电子在衍射中表现出波动行为”。
10. Using Evidence in Exam Questions | 考试题中运用证据
Typical AQA questions ask: 'Describe how this experiment provides evidence for...' or 'Use the graph to determine...' You must state the observation, link it to the model, and avoid unsupported conclusions.
典型的AQA问题会问：“描述该实验如何为……提供证据”或“利用图线确定……”。你必须陈述观察、联系模型，并避免无根据的结论。
- Observation: most alpha particles pass straight through. Conclusion: atom mostly empty space.
- 观察：大多数α粒子直线穿过。结论：原子大部分是空的。
- Observation: very few alpha particles bounce back. Conclusion: nucleus is tiny and positively charged.
- 观察：极少数α粒子反弹。结论：原子核很小且带正电。
- Observation: no photoelectrons below f₀. Conclusion: energy is transferred in discrete quanta.
- 观察：低于 f₀ 没有光电子。结论：能量以离散量子形式传递。
Marks are awarded for precise causal statements, not just naming the effect.
得分点是精确的因果陈述，而不仅仅是说出效应名称。
11. Common Misconceptions | 常见误区
'The photoelectric effect shows that light is a wave.' - No, it provides evidence for the particle nature of light.
“光电效应表明光是一种波。”——不对，它为光的粒子性提供证据。
'Rutherford's experiment proved electrons orbit the nucleus in fixed shells.' - No, it only established the nuclear model; discrete energy levels came later from atomic spectra.
“卢瑟福实验证明电子在固定壳层中绕核运动。”——不对，它只建立了核式模型；分立能级后来来自原子光谱。
'Increasing intensity below the threshold frequency eventually ejects electrons.' - No, no electrons are ejected below f₀ regardless of intensity.
“低于截止频率时增加光强最终会打出电子。”——不对，低于 f₀ 无论光强多大都没有电子发射。
12. Summary Table of Evidence | 证据总结表
The table below summarises the key evidence you must be able to recall and apply in
Published by TutorHao | A-Level Physics Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图