LIGHT LAB / EXPERIMENT 04
The pattern stays.
The wavelengths move.
Light from an approaching source shifts toward shorter wavelengths. Light from a receding source shifts toward longer ones. Match the spectral fingerprint, then read the motion hidden in its displacement.
Explore the motion playground ↓THE MOTION PLAYGROUND
Put the Universe
in motion.
Follow a planet, track glowing gas around a black hole, or stretch a galaxy’s arriving light. Scrub a scene yourself or press Play, then compare what Earth receives.
A planet goes around. Its shift comes and goes.
This fictional planet carries a narrow 550 nm test-light beacon. Follow it toward Earth, sideways and away. Real planetary motion produces a tiny color change; the enhanced view makes its sign easy to follow.
One solar-mass star, circular orbit, observer far away along the diagram’s toward-Earth axis. The diagram compresses distance, exaggerates the planet and ignores light-travel delay. A beacon isolates the Doppler lesson; reflected planetary starlight involves an additional illumination geometry.
The gas orbits. Its light escapes.
The approaching and receding parts of a glowing ring carry different wavelength shifts. Follow one bright gas clump, change its speed and tilt the ring to see the pattern change.
This is a Doppler-only kinematic ring, not a general-relativistic disk image. Gas moves around the black hole; light does not turn red and blue simply by circling it. Photon paths, gravitational redshift, lensing, beaming, disk temperature and physical orbital dynamics are omitted. The black center is a diagram symbol. The colored dotted ray marks the selected emitter’s received shift, not a traced photon orbit.
Follow the light from then to now.
Each galaxy starts with an illustrative emission-epoch example. Select one and stretch its light. Strong cosmological redshift can carry visible light beyond red into infrared.
The galaxy positions are illustrative, not distances or ages. Different example redshifts are assigned different emission scale factors, not inferred from their placement. Expansion playback interpolates scale factor for teaching; it does not calculate a cosmological history or flight time. Galaxy tints show arriving test light, not their intrinsic colors. Local motion is a separate experiment.
Playback compresses time. It starts only when you choose Play and stops when this playground is offscreen or the tab is hidden. All comparisons also work with the phase slider.
EARTH’S DETECTOR / TEST LIGHT
550 nm at the source → the selected received wavelength.
- Calculated wavelength shift
- -0.000086
- Line-of-sight motion
- -25.79 km/s
- Orbital speed / period
- 29.78 km/s · 365.26 days
The guide uses a logarithmic wavelength axis. Approximate visible limits are 380–750 nm. In test-line color mode, UV/infrared use gray markers because screens cannot display those wavelengths.
The starting planet snapshot is shown. Loading playground controls…
Read the color key before interpreting the sky.
Enhanced red/blue colors encode wavelength shift with different contrast scales for planets, gas and galaxies. They make subtle shifts easy to see and are not a comparison of shift magnitude between scenes. The test-line view approximates only one 550 nm emission line, not the combined true color or brightness of an astronomical object.
For the orbit scenes, the wavelength factor is γ(1 + vaway/c), using the full source speed in γ and its projected line-of-sight velocity in the second factor. A face-on moving ring has zero radial motion but retains a transverse relativistic redshift. Real observations also include gravity and other corrections. Here the observer lies along positive depth, so the right-hand orbit point initially moves toward Earth.
For expanding space, 1 + z = aobserved/aemitted. Today’s scale factor is set to 1. Earlier emission means more stretching in these selected examples; an object’s drawn position does not determine z, age, distance or a local Doppler velocity. A nearby galaxy’s local motion can produce a blueshift without contradicting cosmic expansion.
Compare precise spectral lines below ↓REST / RECEIVED / RADIAL MOTION
One source. A shifted signature.
The laboratory reference is a source at rest relative to the observer. Both spectra use the same wavelength axis. Positive velocity means moving away from Earth.
Static example: a source receding at 3,000 km/s. Loading interactive controls…
Changing wavelength, constant light speed.
Receding · redshift
A velocity arrow shows the source’s motion at one instant. Wave drawings below compare spacing; source and Earth positions are schematic.
Compare the wave spacing
Wave spacing uses one common illustration scale. Animation slows light down enormously and illustrates the same propagation speed in each frame, with frequency changing inversely with wavelength. No travel time, source trajectory, flux or physical wave amplitude is calculated. Playback is optional; the static comparison contains the same information.
Match the whole pattern
Dashed gray: rest reference · Solid mint: received
Shaded ends lie outside the approximate 380–750 nm visible guide. A redshift can move light into infrared, and a blueshift into ultraviolet. Both normalized spectral shapes deliberately omit relativistic brightness changes. Artificially broadened lines are teaching profiles, not predictions for a real atmosphere or instrument.
READ THE DISPLACEMENT
A small shift needs a closer look.
Selected line: Hα. Ruler auto-fits both wavelengths; its tick labels change with the range.
Hα: 656.5 → 663.1027 nm. Ruler auto-fits both wavelengths.
| Line | Rest (nm) | Received (nm) | Shift (nm) |
|---|---|---|---|
| 410.3 | 414.427 | 4.127 | |
| 434.2 | 438.567 | 4.367 | |
| 486.3 | 491.191 | 4.891 | |
| 656.5 | 663.103 | 6.603 |
- Direction along the line of sight
- Receding · redshift
- Redshift z = (λ received / λ rest) − 1
- 0.010057
- Received / rest wavelength
- 1.010057×
- Received / rest frequency
- 0.990043×
FOLLOW THE REASONING
A shift has to match a pattern.
Identify several transitions and compare their received wavelengths with laboratory references. In this ideal radial-motion model, every wavelength is multiplied by the same factor. The pattern stretches or compresses; it does not slide by one constant number of nanometers. A single unidentified line could be mistaken for another transition, so real spectra require context, calibration and multiple features.
Motion and temperature answer different questions.
Use At rest, then change the temperature. The continuum shape changes while the line centers stay fixed. Choose Receding or Approaching and the entire spectral pattern shifts. A cool source can look red without receding, and extinction can also redden light. A spectrum gives more information than a visible color alone. Here the temperature affects only an ideal Planck continuum; excitation, ionization, line strengths and widths remain teaching assumptions. The emission mode therefore disables the continuum temperature control.
The calculation and its limits.
For purely radial inertial motion in flat spacetime, λ received / λ rest = √((1 + β) / (1 − β)), where β = radial velocity / c. Positive velocity means recession. Frequency changes by the reciprocal factor. The exact vacuum speed of light is 299,792.458 km/s, about 186,282.4 miles/s. From z, the inverse relation is β = ((1 + z)² − 1) / ((1 + z)² + 1). The familiar v ≈ cz is only a low-speed approximation.
This lab spans −60,000 to +60,000 km/s, about ±20% of c, to make shifts easy to see. The large-speed examples are illustrative and do not describe typical stellar radial velocities. This original radial-spectrum model excludes transverse motion, orbital acceleration, gravitational shifts, cosmological expansion, instrumental noise and relativistic beaming. Hydrogen wavelengths are rounded vacuum values to 0.1 nm; they are unsuitable for precision velocity work. The close-up reveals calculated shifts, not an instrument’s resolving power.
Redshift does not supply a universal distance.
Nearby relative motion, gravitational fields and the expansion of the Universe can all shift wavelengths. This control models local radial Doppler motion only. A cosmological redshift requires an expansion model and other assumptions to relate it to distance; substituting it into this local velocity equation would answer a different question. Parallax explores geometric distance directly.
Check the source trail.
SDSS / Vacuum wavelength convention ↗
SDSS tabulates vacuum hydrogen references and the air-to-vacuum conversion. The four references here are rounded to 0.1 nm.
OpenStax / Relativistic Doppler effect ↗
The wavelength relation includes special relativity and uses positive radial velocity for a receding source.
NASA / Doppler shift of a star’s spectrum ↗
Compare a known spectral pattern with its shifted counterpart to infer motion along the line of sight.
NASA / Beyond temperature and composition ↗
Spectra carry information about motion and other physical processes; cosmological redshift requires a different interpretation.
NIST / Hydrogen line data ↗
Hydrogen’s Balmer pattern motivates these rounded vacuum teaching references. This model does not resolve fine structure or reproduce calibrated line strengths.
NASA / Cosmological redshift ↗
Expansion stretches wavelengths during their journey. This lab’s local radial-motion formula is not a cosmological distance calculator.
Keep reading the light.
Build a spectrum and identify atomic fingerprints · Measure parallax · Follow gravitationally bent light · Separate measurements from their interpretation
SCIENTIFIC INQUIRY / APPLIED HERE
Explore all twelve principlesA wavelength shift needs an identified pattern.
Match several transitions to reference wavelengths and test whether their wavelength ratios agree. The displayed speed assumes purely radial Doppler motion in flat spacetime; cosmological expansion and gravity require different models. A teaching animation supplies no measured spectrum or distance.
Ask yourself Which assumptions drive this result, and where would the model stop being useful?