Doppler Shift of Light

Name: ________________________________ Section: __________ Date: __________

Station: __________ Group members: ________________________________________________

Goal: infer motion from spectral evidence and justify when relativity matters.

Station card: Doppler Shift (10-12 minutes) Artifact: one comparison table + one model-choice justification.

1. Start at rest and record one representative line for your selected element.
2. Set $v_r=+300\ \text{km/s}$ and then $v_r=-300\ \text{km/s}$.
   • Record $\lambda_{\rm obs}$, $\Delta\lambda$, $\nu_{\rm obs}$, and shift direction.
3. Switch to presets 7 and 8.
   • Compare non-rel and relativistic predictions.
   • Use the red/blue marker thresholds on the $z$ slider and record the NR error percentage.
4. Use the Sound vs Light callout.
   • Explain why the wave crests in this demo are uniformly spaced at the observer.
5. Tap Why this line? and note how representative-line anchoring is chosen.
6. Run one Mystery Spectrum round (predict -> check -> explain), then use Copy challenge evidence.
7. Write one sentence claim:
   • “For this case, the non-rel formula is / is not acceptable because _____.”

Case	Element	$v_r$ (km/s)	$z_{\rm nonrel}$	$z_{\rm rel}$	$\lambda_{\rm obs}$ (nm)	$\Delta\lambda$ (nm)	NR error (%)
Rest
+300 km/s
-300 km/s
Preset 7
Preset 8

Sanity checks

• Receding sources shift to longer wavelength (redshift); approaching sources shift to shorter wavelength (blueshift).
• Non-rel and relativistic formulas should agree closely at low speed.
• At high redshift, the non-rel formula should visibly diverge.
• Kinematic Doppler redshift is not the same mechanism as cosmological or gravitational redshift.