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WO · Waves and optics

GRE · GRE Subject Test · GRE 物理 · 知识点 3

训练
3.1

波动与光学

Two individually bright waves can cancel at a point if their phases oppose.

Prerequisites: 16, 17, 18, 31, 32, 33.

  • Analyse wave superposition and interference
  • Use diffraction 衍射, polarisation and optical imaging
  • Relate phase, frequency and dispersion
词汇 训练
English 中文 拼音
diffraction/dɪˈfrækʃn/ 衍射 yǎn shè
3.2

Choose the system and model

Add wave amplitudes, then calculate intensity. Intensity is proportional to squared amplitude for the same medium.

3.3

Use the governing relation

A path difference gives phase difference 相位差 2πΔ/λ. Coherent sources need stable relative phase.

词汇 训练
English 中文 拼音
phase difference/feɪz ˈdɪfrəns/ 相位差 xiàng wèi chā
3.4

Apply the conditions

For a single slit, minima satisfy a sin θ=mλ for nonzero integers m. A double slit has a different interference condition.

3.5

Check the conclusion

Geometric optics needs a stated sign convention. Wave effects matter when apertures are comparable to wavelength.

3.6

Worked method

Coherent waves have a stable phase difference. Add amplitudes before squaring.

$$I=I_1+I_2+2\sqrt{I_1I_2}\cos\delta.$$
For equal individual intensities I0 and a path difference of one quarter wavelength,
$$\delta=2\pi\Delta/\lambda=2\pi(\lambda/4)/\lambda=\pi/2.$$
$$I=2I_0(1+\cos\delta)=2I_0(1+\cos(\pi/2))=2I_0.$$
This is neither a maximum nor a minimum. Random relative phase removes the cross term in a long time average; stable coherence 相干性 is a physical condition, not a drawing convention.

Waves and optics: GRE original diagram
Waves and optics: original GRE teaching diagram.
词汇 训练
English 中文 拼音
coherence/kəʊˈhɪərəns/ 相干性 xiāng gān xìng
3.7

Check conditions and vocabulary

The central single-slit maximum is not obtained by setting m=0 in the minima condition.

coherence: A stable phase relationship between waves.

diffraction: Wave spreading and interference caused by an aperture or obstacle.

该知识点的互动课程

逐步完成，配合即时检查练习。

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