IB Physics: Doppler Effect Principle & Formula Applications | IB物理：多普勒效应原理与公式应用

📚 IB Physics: Doppler Effect Principle & Formula Applications | IB物理：多普勒效应原理与公式应用

The Doppler effect describes the apparent change in frequency (and wavelength) of a wave when there is relative motion between the source and the observer. It is a fundamental concept in wave physics and appears in both sound and electromagnetic waves.

多普勒效应描述了当波源与观察者之间存在相对运动时，波的表观频率（和波长）发生变化的现象。它是波动物理学中的基本概念，既适用于声波，也适用于电磁波。

1. What Is the Doppler Effect? | 什么是多普勒效应？

The Doppler effect is named after Austrian physicist Christian Doppler, who proposed it in 1842. When a wave source moves toward an observer, the waves are compressed, leading to a higher observed frequency. When the source moves away, the waves are stretched, leading to a lower observed frequency.

多普勒效应以奥地利物理学家克里斯蒂安·多普勒命名，他于1842年提出这一概念。当波源向观察者移动时，波被压缩，导致观察频率升高；当波源远离观察者时，波被拉伸，导致观察频率降低。

A common example is the change in pitch of a siren as an ambulance passes by. As it approaches, the pitch sounds higher; after it passes, the pitch drops.

一个常见的例子是救护车驶过时警笛音调的变化。当它靠近时，音调听起来更高；经过后，音调降低。

2. Wavefronts and Relative Motion | 波前与相对运动

To understand the Doppler effect, imagine a source emitting circular wavefronts. If the source is stationary, the wavefronts are concentric circles. If the source moves, the wavefronts become closer together in front of the source and farther apart behind it.

要理解多普勒效应，想象一个波源发出圆形波前。如果波源静止，波前是同心圆。如果波源移动，波源前方的波前会变得更密集，后方的波前则更稀疏。

Observer stationary, source moving: The wavelength in front of the source is shortened to λ’ = λ − v_s T.
观察者静止、波源运动：波源前方的波长缩短为 λ’ = λ − v_s T。
Source stationary, observer moving: The observed frequency changes because the observer encounters wavefronts at a different rate.
波源静止、观察者运动：观察者以不同的速率遇到波前，因此观察频率改变。

3. Sound Doppler Formula: General Case | 声波多普勒公式：一般情况

For sound waves, the Doppler formula in its most general form is:

f’ = f × (v ± vₒ) / (v ∓ vₛ)

where:

f’ = observed frequency (Hz)
f = emitted frequency (Hz)
v = speed of sound in the medium (m/s)
vₒ = speed of the observer relative to the medium (m/s)
vₛ = speed of the source relative to the medium (m/s)

其中：

f’ = 观察到的频率（Hz）
f = 发射频率（Hz）
v = 声波在介质中的速度（m/s）
vₒ = 观察者相对于介质的速度（m/s）
vₛ = 波源相对于介质的速度（m/s）

The upper signs (+vₒ and −vₛ) are used when the observer and source move toward each other. The lower signs (−vₒ and +vₛ) are used when they move apart.

当观察者与波源相向运动时使用上面的符号（+vₒ 和 −vₛ）；当它们彼此远离时使用下面的符号（−vₒ 和 +vₛ）。

4. Special Cases: Moving Source or Moving Observer | 特殊情况：波源运动或观察者运动

When only the source moves (observer stationary, vₒ = 0):

当只有波源运动（观察者静止，vₒ = 0）时：

f’ = f × v / (v ∓ vₛ)

Use the minus sign when the source approaches, plus when it recedes.

波源接近时用减号，远离时用加号。

When only the observer moves (source stationary, vₛ = 0):

当只有观察者运动（波源静止，vₛ = 0）时：

f’ = f × (v ± vₒ) / v

Use the plus sign when the observer approaches, minus when receding.

观察者接近时用加号，远离时用减号。

5. Worked Example: Sound Siren | 例题：警笛声

Example: A police car siren emits a frequency of 1000 Hz. The car moves at 30 m/s toward a stationary observer. The speed of sound is 340 m/s. What frequency does the observer hear?

例题： 警车警笛发出频率为1000 Hz的声音。警车以30 m/s的速度向静止的观察者行驶。声速为340 m/s。观察者听到的频率是多少？

Using f’ = f × v / (v − vₛ), because the source is approaching:

使用 f’ = f × v / (v − vₛ)，因为波源接近：

f’ = 1000 × 340 / (340 − 30) = 1000 × 340 / 310 ≈ 1096.8 Hz

The observed frequency is higher than the emitted frequency, as expected.

观察到频率高于发射频率，与预期相符。

6. Electromagnetic Doppler Effect | 电磁波的多普勒效应

For electromagnetic waves (light, radio, etc.), the Doppler effect must be treated using special relativity. For speeds much less than the speed of light c, the approximate formula is:

对于电磁波（光、无线电等），必须使用狭义相对论来处理多普勒效应。当速度远小于光速 c 时，近似公式为：

f’ ≈ f (1 ± v/c)

More exactly, for an observer moving directly toward or away from a source, the relativistic Doppler formula is:

更精确地，对于观察者直接朝向或远离波源运动，相对论性多普勒公式为：

f’ = f × √[(1 + β) / (1 − β)]

where β = v/c, and v is the relative speed. This formula accounts for time dilation.

其中 β = v/c，v 是相对速度。该公式考虑了时间膨胀效应。

Redshift: When a source moves away, the frequency decreases and wavelength increases (shifts toward red). This is the basis for measuring the expansion of the universe.
红移： 当波源远离时，频率降低，波长增大（向红色端移动）。这是测量宇宙膨胀的基础。
Blueshift: When a source moves toward us, the frequency increases and wavelength decreases.
蓝移： 当波源向我们靠近时，频率增加，波长减小。

7. Applications in Medicine: Ultrasound | 医学应用：超声

Doppler ultrasound is used to measure the speed of blood flow. A probe sends ultrasound waves into the body; the waves reflect off red blood cells. The reflected wave has a Doppler-shifted frequency depending on the velocity of the cells.

多普勒超声用于测量血流速度。探头向体内发射超声波；超声波被红细胞反射。反射波的频率根据细胞速度发生多普勒频移。

Δf = f’ − f = (2 f v cos θ) / c

where v is the blood velocity, θ is the angle between the ultrasound beam and the flow direction, and c is the speed of ultrasound in tissue.

其中 v 是血流速度，θ 是超声波束与血流方向之间的夹角，c 是超声在组织中的速度。

8. Astronomical Applications: Radial Velocity | 天文应用：径向速度

Astronomers measure the Doppler shift of spectral lines to determine the radial velocity of stars and galaxies. The formula for radial velocity is:

天文学家测量光谱线的多普勒频移以确定恒星和星系的径向速度。径向速度公式为：

v = c × Δλ / λ₀

where Δλ = λ − λ₀ is the shift in wavelength from the rest wavelength λ₀.

其中 Δλ = λ − λ₀ 是波长相对于静止波长 λ₀ 的偏移。

Exoplanet detection: A star’s periodic radial velocity wobble, caused by an orbiting planet, produces periodic Doppler shifts in its spectrum.
系外行星探测： 行星绕恒星运动使恒星产生周期性的径向速度摆动，从而在光谱中产生周期性的多普勒频移。
Cosmology: The redshift of distant galaxies is proportional to their distance (Hubble’s law), providing evidence for the expanding universe.
宇宙学： 遥远星系的红移与其距离成正比（哈勃定律），为宇宙膨胀提供了证据。

9. Solving IB Doppler Problems: Step-by-Step | 解IB多普勒题：逐步指南

Follow these steps to avoid sign errors:

按以下步骤避免符号错误：

Identify the type of wave. Sound uses the classic formula; light uses the relativistic formula.
明确波的种类。 声波使用经典公式；光使用相对论公式。
Define positive direction. Usually, take the direction from observer to source as positive.
确定正方向。 通常，取从观察者指向波源的方向为正。
Determine whether the distance between source and observer is decreasing or increasing.
判断波源与观察者之间的距离是减小还是增大。
Choose the correct signs in the formula. If distance decreases, the observed frequency must be higher.
在公式中选择正确的符号。 如果距离减小，观察到的频率必须更高。
Check units and convert to SI. Speeds should be m/s, frequencies in Hz.
检查单位并换算为国际单位制。 速度应为 m/s，频率应为 Hz。
If necessary, compute the wavelength using λ = v/f.
如需要，用 λ = v/f 计算波长。

10. Common Pitfalls and Misconceptions | 常见错误与误解

Using the same sign for both source and observer. The signs are different: one uses plus, the other minus.
对波源和观察者使用相同的符号。 它们的符号不同：一个用加，另一个用减。
Confusing the direction of motion with the sign. Always think about whether the distance is increasing or decreasing.
混淆运动方向与符号。 始终思考距离是增大还是减小。
Applying the sound formula to light at high speeds. The classical formula is not accurate for light; use the relativistic form.
在高速情况下将声波公式应用于光。 经典公式对光不准确；应使用相对论形式。
Forgetting that the medium matters. For sound, the speeds are measured relative to the medium (air), not relative to each other.
忘记介质的影响。 对于声波，速度是相对于介质（空气）测量的，而不是相对彼此。

11. Quick Formula Summary | 公式速查总结

Situation	Formula
Source moving, observer stationary	f’ = f v / (v ∓ vₛ)
Observer moving, source stationary	f’ = f (v ± vₒ) / v
General sound case	f’ = f (v ± vₒ) / (v ∓ vₛ)
Electromagnetic (low speed)	f’ ≈ f (1 ± v/c)
Relativistic Doppler	f’ = f √[(1 ± β)/(1 ∓ β)]

Remember: For approaching, use the sign that increases frequency; for receding, use the sign that decreases frequency.

记住：接近时，使用使频率增大的符号；远离时，使用使频率减小的符号。

12. Exam Tips for IB Physics | IB物理考试技巧

In IB Physics Paper 2, Doppler effect questions often ask you to calculate observed frequency or explain a real-world scenario. You should be able to derive the simple formula from the wavefront diagram, not just memorize it.

在IB物理Paper 2中，多普勒效应问题通常要求你计算观察频率或解释现实场景。你应该能够从波前图推导出简单公式，而不只是记忆。

Draw wavefront diagrams to visualise the compression and rarefaction.
画出波前图以可视化压缩与稀疏。
Always state the direction of motion when describing the effect.
描述效应时始终说明运动方向。
For electromagnetic waves, mention the relativistic factor if the speed is a significant fraction of c.
对于电磁波，如果速度是光速的显著比例，请提及相对论因子。

Practice with past paper questions, especially those involving units and sign conventions. Mastery of Doppler effect will not only earn you marks but also deepen your understanding of wave interference and relativity.

多做真题练习，尤其是涉及单位和符号约定的题目。掌握多普勒效应不仅能帮你得分，还能加深你对波干涉和相对论的理解。

Published by TutorHao | Physics Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

IB Physics: Doppler Effect Principle & Formula Applications | IB物理：多普勒效应原理与公式应用