Quantitative description of frequency shifts (HL)
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Flip to reveal answersWhat does the HL part of the Doppler effect add over SL?
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All 12 Flashcards — Quantitative description of frequency shifts (HL)
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Question
What does the HL part of the Doppler effect add over SL?
Answer
The **calculation** of the shift: the observed frequency f' (sound) or the fractional shift Δf/f (light), not just the qualitative 'higher/lower'.
Question
Moving-source (sound) Doppler equation?
Answer
$f' = f\,\dfrac{v}{v \mp u_s}$ — use **−** approaching, **+** receding.
Question
In f' = f·v/(v ∓ u_s), which sign for an approaching source?
Answer
The **minus** sign (smaller denominator) ⇒ bigger fraction ⇒ **higher** f'.
Question
Light Doppler shift for v ≪ c?
Answer
$\dfrac{\Delta f}{f} = \dfrac{\Delta\lambda}{\lambda} \approx \dfrac{v}{c}$.
Question
What does a redshift tell you?
Answer
The wavelength is **longer**, so the source is **receding** (moving away).
Question
What does a blueshift tell you?
Answer
The wavelength is **shorter**, so the source is **approaching**.
Question
Why doesn't the sound equation work for light?
Answer
Light needs **no medium**, so there is no medium speed v of sound; use Δλ/λ ≈ v/c instead.
Question
When is Δλ/λ ≈ v/c valid?
Answer
Only when the source speed is **much less than c** ($v \ll c$); it is an approximation.
Question
Siren 512 Hz approaches at 30 m s⁻¹, v = 340 m s⁻¹: f'?
Answer
$f' = 512 \times \dfrac{340}{340-30} = 561$ Hz (higher).
Question
Galaxy with Δλ/λ = 0.02: recession speed?
Answer
$v = c \times 0.02 = 6 \times 10^{6}$ m s⁻¹ (about 0.02c).
Question
First check before any Doppler calculation?
Answer
Decide **physically**: approaching ⇒ higher f' (red/blue: blueshift = approaching). Then match the algebra to it.
Question
Sound vs light Doppler — what does each give?
Answer
Sound gives the **actual** f'; light gives the **fractional** shift Δλ/λ (multiply by λ or f for the change).
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