14.5 The Doppler Effect

The Doppler effect is the change in the frequency (or pitch) you observe when a wave source and an observer move relative to each other. If the source and observer have no relative motion, you hear the source’s rest frequency. If the source moves toward you, wavefronts get compressed so you measure a higher frequency (pitch goes up, “blueshift” for light). If it moves away, wavefronts stretch and you measure a lower frequency (pitch goes down, “redshift”). The faster the relative (radial) speed, the bigger the shift. For AP Physics 2 you only need a qualitative understanding: identify which way the frequency changes and connect that to relative velocity and wavefront compression/expansion (CED 14.5.A.1–A.2). Practice recognizing scenarios on the exam and explain using “toward = higher, away = lower.” For a focused review see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz), the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14), and try AP-style practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

When an ambulance moves toward you, each successive sound wave crest is emitted from a position closer to you than the previous crest, so the crests get bunched up (wavefront compression). That means you encounter more crests per second—a higher observed frequency—so the siren sounds higher in pitch. When it moves away, the crests are stretched out (wavefront expansion), you encounter fewer crests per second, and the pitch is lower. The size of the pitch shift depends on the relative velocity between you and the source: larger relative speed → bigger difference between observed and rest (source) frequency (CED 14.5.A.1–A.2). For AP Physics 2 you only need this qualitative explanation (no relativistic math). For a short study refresher see the Topic 14.5 Doppler Effect guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and try extra practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

The Doppler effect is just how the frequency (or pitch/color) you observe changes when the source and observer move relative to each other. If they move together at the same speed you measure the rest frequency. If the source moves toward you (or you move toward the source) the wavefronts get compressed—you observe a higher frequency (blueshift / higher pitch). If they move apart the wavefronts stretch—you observe a lower frequency (redshift / lower pitch). The size of the shift depends on the relative velocity: bigger relative speed → bigger change in observed vs. rest frequency. For AP Physics 2 you only need qualitative descriptions like this (no relativistic formulas) and to link wavefront compression/expansion to observed frequency changes (CED 14.5.A). For a short study guide on Topic 14.5 see Fiveable (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz). For unit review and lots of practice problems, check the unit page (https://library.fiveable.me/ap-physics-2-revised/unit-14) and practice hub (https://library.fiveable.me/practice/ap-physics-2-revised).

Rest (or source) frequency is the frequency the source actually emits—what you'd measure if you sat right next to the source with no relative motion. Observed frequency is what a receiver hears or detects; it can differ because of relative motion between source and observer. If the source and observer move together (same velocity along the line of sight) the observed frequency equals the rest frequency. If the source moves toward you, wavefronts get compressed so the observed frequency is higher (blueshift/pitch up); if it moves away, wavefronts stretch so the observed frequency is lower (redshift/pitch down). The CED requires only a qualitative relationship: larger relative speed → larger difference between observed and rest frequencies (14.5.A.1–A.2). For a quick AP review, see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and try practice problems (https://library.fiveable.me/practice/ap-physics-2-revised) to get comfortable with wording on the exam.

If the source and observer move relative to each other, the observed frequency (what you hear) changes from the source’s rest frequency. Qualitatively: if they move toward each other the observed frequency is higher (wavefronts get compressed → higher pitch, “blueshift”); if they move away the observed frequency is lower (wavefronts expand → lower pitch, “redshift”). The bigger the relative (radial) speed, the bigger the difference between observed and rest frequency; if source and observer move together at the same velocity, the observed frequency equals the rest frequency. AP only needs this qualitative picture (wavefront compression/expansion, pitch shift)—no relativistic math unless asked. For a quick review, see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14). Practice more Doppler-style questions at (https://library.fiveable.me/practice/ap-physics-2-revised).

When a source moves toward you, each successive wave crest is emitted from a position a little closer to you than the previous one. That makes the crests bunch up in front of the source (wavefront compression), so more crests pass your ear per second—i.e., you measure a higher frequency than the source’s rest frequency. In AP terms: the observed frequency > rest frequency when the source moves toward the observer (14.5.A.2.ii). The amount the frequency shifts grows with the relative speed between source and observer (14.5.A.2). For sound this is heard as a higher pitch (a “blueshift” term is used for light). AP Physics 2 only needs a qualitative explanation like this on the exam. For a quick refresher, see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz). For more review and practice, check Unit 14 overview (https://library.fiveable.me/ap-physics-2-revised/unit-14) and the practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

If the source and observer move at the same velocity (so their relative velocity is zero along the line connecting them), there’s no Doppler shift—the observer hears the rest (emitted) frequency. In other words, equal speeds in the same direction means the wavefronts aren’t being compressed or stretched relative to the observer, so observed frequency = source (rest) frequency (CED 14.5.A.2.i). If the source moves toward the observer (relative velocity toward > 0) you get a higher (blueshifted) frequency; if it moves away you get a lower (redshifted) frequency (CED 14.5.A.2.ii/iii). AP Physics 2 only requires this qualitative idea—focus on relative velocity, not algebraic relativistic formulas. For a quick review, see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and practice questions (https://library.fiveable.me/practice/ap-physics-2-revised).

Lots of examples—the Doppler effect shows up in sound, medical tech, and astronomy. For sound: a passing train or motorcycle horn changes pitch (higher as it approaches, lower as it recedes) and bats use frequency shifts in echoes to judge prey distance/speed. In policing and traffic, radar speed guns measure vehicle radial velocity via Doppler shift of microwaves. Weather Doppler radar detects wind speed and storm rotation by measuring frequency shifts of returned radio waves. In medicine, Doppler ultrasound measures blood flow speed and direction. In astronomy, galaxy spectra show redshift (moving away) or blueshift (moving toward), which is a Doppler effect for light and tied to radial velocity and the expanding universe. Remember AP focuses on qualitative relationships: relative motion changes observed frequency (blueshift if toward, redshift if away) and greater relative speed gives a bigger shift (CED 14.5.A). For quick review see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and extra practice (https://library.fiveable.me/practice/ap-physics-2-revised).

You're right that the source itself keeps emitting at its rest frequency f0. The Doppler effect happens because motion changes the spacing of the wavefronts that reach you, so the wavelength you receive (λobs) is different even though the source's emission rate is the same. If the source moves toward you, successive wave crests are emitted closer together (wavefront compression) so λobs is smaller → your observed frequency fobs = vwave/λobs is larger (blueshift). If the source moves away, wavefronts are stretched, λobs is larger → fobs is smaller (redshift). The same logic applies if the observer moves relative to the medium—only the relative velocity matters (CED 14.5.A). AP only needs this qualitative idea: rest frequency vs observed frequency and that greater relative speed gives a bigger shift (14.5.A.1–A.2). For a clear visual and more practice, check the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and try more problems at Fiveable practice (https://library.fiveable.me/practice/ap-physics-2-revised).

Quick trick: if the source and observer move toward each other you get a higher observed frequency (pitch up / blueshift); if they move away you get a lower frequency (pitch down / redshift). Physically: moving toward compresses wavefronts (shorter wavelength → higher f), moving away stretches them (longer wavelength → lower f). Memory aids: - Imagine an ambulance: approaching = higher pitch, receding = lower pitch. - Phrase: “Toward = Tight (compressed) → Toned-up (higher f); Away = Away (expanded) → Down (lower f).” Remember AP Physics 2 only needs qualitative reasoning (CED 14.5.A: toward → fobs > f0; away → fobs < f0). For a refresher, see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and more unit review (https://library.fiveable.me/ap-physics-2-revised/unit-14). Practice problems are on Fiveable too (https://library.fiveable.me/practice/ap-physics-2-revised).

Short answer: the faster the source and observer move relative to each other, the bigger the change in observed frequency. If they move together at the same velocity you hear the rest (source) frequency; if the source moves toward you the observed frequency is higher (blueshift/pitch up); if it moves away the observed frequency is lower (redshift/pitch down). The magnitude of the shift grows as the relative (radial) speed increases—greater relative velocity → greater difference between observed and rest frequencies (CED 14.5.A.1–A.2). For AP Physics 2 you only need a qualitative picture, but if you want the usual nonrelativistic idea: the observed-to-rest frequency ratio depends on (sound or light speed ± observer or source speed), so larger speeds give larger fractional shifts. Review the Topic 14.5 study guide for this (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and try practice problems (https://library.fiveable.me/practice/ap-physics-2-revised).

Short answer: because the AP Physics 2 CED only asks you to describe how observed frequency changes with relative motion, not to derive or compute Doppler formulas. The boundary statement for Topic 14.5 says “Only qualitative treatments…are required.” That means you need to be able to use AP vocabulary—rest (source) frequency, observed frequency, relative velocity, redshift/blueshift, wavefront compression/expansion—and make correct predictions: if source moves toward you observed frequency > rest frequency (blueshift/pitch up); away → observed frequency < rest (redshift/pitch down); greater relative speed → bigger shift; same velocity → no shift (14.5.A.1–A.2). You don’t need the quantitative classical or relativistic Doppler equations on the exam, but you should be ready for MCQs or FR parts that ask you to compare scenarios or explain qualitatively (Topic 14 is ~12–15% of MCQs). Review the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz), the unit overview (https://library.fiveable.me/ap-physics-2-revised/unit-14), and practice problems (https://library.fiveable.me/practice/ap-physics-2-revised) to practice the qualitative reasoning the exam expects.

Short answer: it works for all kinds of waves—not just sound. The Doppler effect is about how the observed frequency changes when the source and observer have relative (radial) motion, so any wave with a well-defined frequency can show a Doppler shift: sound, water waves, and electromagnetic waves (light, radio, etc.). On the AP Physics 2 CED this is Topic 14.5: you only need a qualitative treatment—more relative speed means a bigger shift; toward you = higher observed frequency (blueshift for light), away = lower (redshift). For light at high speeds you’d use the relativistic Doppler ideas (mentioned in the CED keywords). For practice and review, check the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and the Unit 14 overview (https://library.fiveable.me/ap-physics-2-revised/unit-14). For more practice problems, see Fiveable’s practice page (https://library.fiveable.me/practice/ap-physics-2-revised).

Look at the observed frequency fobs compared to the source’s rest frequency f0: - If fobs > f0 → source is moving toward the observer (blueshift/higher pitch). - If fobs < f0 → source is moving away from the observer (redshift/lower pitch). - If fobs = f0 → no relative radial motion (same velocity along the line of sight). Also: the bigger the difference |fobs − f0|, the larger the relative speed along the line of sight (CED 14.5.A.1–A.2). Example: a 440 Hz siren heard at 460 Hz means the source is approaching; heard at 420 Hz means it’s receding. Note this is qualitative AP-level treatment—you don’t need relativistic formulas on the AP exam. For a quick review, check the Topic 14.5 Doppler study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz) and more unit resources at (https://library.fiveable.me/ap-physics-2-revised/unit-14). For practice problems, use Fiveable’s practice page (https://library.fiveable.me/practice/ap-physics-2-revised).

If the source and observer move in the same direction, what matters is their relative velocity. Qualitatively: the observed frequency changes depending on whether the observer is closing the gap or letting it widen. - If v_observer = v_source, relative speed = 0 → observed frequency = rest (no Doppler shift) (CED 14.5.A.2.i). - If the observer moves faster than the source (closing the gap), you hear a higher frequency (blueshift) (CED 14.5.A.2.ii). - If the source moves faster than the observer (gap widens), you hear a lower frequency (redshift) (CED 14.5.A.2.iii). AP only requires this qualitative reasoning about relative velocity and whether the gap is increasing or decreasing (Topic 14.5). For a quick refresher, see the Topic 14.5 study guide (https://library.fiveable.me/ap-physics-2-revised/unit-6/5-the-doppler-effect/study-guide/Qhw4LQ7MLNCPQArz). For more practice, check the unit practice set (https://library.fiveable.me/practice/ap-physics-2-revised).