Spectral shifts: a low-speed astronomical estimate
Введено| English | Значение |
|---|---|
| redshift/ˈredʃɪft/ | Красное смещение |
| rest wavelength/rest ˈweɪvleŋθ/ | Длина волны покоя |
What would explain this observation?
- A recognizable spectral line appears at a longer wavelength in a distant source. Its shift can provide evidence of recession along the line of sight.
Build the model
- Relative source-observer motion changes observed frequency or wavelength.
- For light at a relative speed much smaller than c, the magnitude of the fractional wavelength shift is approximately v/c.
- Recession corresponds to longer wavelength, and approach to shorter wavelength.
- redshift 红移: Shift of an identified line toward longer wavelength.
- rest wavelength 静止波长: Wavelength of the line measured with no relative source motion.
What does an identified line shifting to a longer wavelength suggest in the stated model?
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
A known line found at a longer wavelength is redshifted, which in this model means the source is receding along the line of sight. The speed of light is constant, and a shifted line still has a nonzero frequency.
Match each technical term to its precise meaning.
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Compare a identified line with its laboratory rest wavelength.
- Use z = (observed−rest)/rest, then v approximately cz in the stated low-speed model.
- Do not apply this approximation without checking the regime or treating a cosmological redshift as a simple exact velocity.
In the lab: Use attributed spectra with calibration and line identification. Compare several lines for a consistent shift. State uncertainty and distinguish relative line-of-sight motion from an unmeasured transverse component.
Which two habits make the investigation or model in this case more defensible?
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: a 500 nm rest line is observed at 501 nm. z = (501−500)/500 = 0.002.
- With c=3.0×10⁸ m/s, v approximately 6.0×10⁵ m/s away.
- The sound-source denominator formula is not the required SL light approximation.
Your turn: A 600 nm rest line is observed at 601.2 nm. Find z=(observed−rest)/rest.
A 600 nm rest line is observed at 601.2 nm. Find z=(observed−rest)/rest.
Сложность 2/5 · 140 монет + небольшой бонус за первое выполнение сегодня
The result is 0.002 . Known: rest 600 nm; observed 601.2 nm. Use z = (observed - rest)/rest. z = (601.2 - 600)/600 = 0.002. Positive z is a redshift; at low speeds v is approximately cz.
Check the conclusion and its limits
- The approximate shift relation does not show that light travels faster from an approaching source.
- A single unidentified line cannot safely establish a redshift.
Not true: An observed redshift means light speed increased. Light speed in vacuum is the same for all observers. Redshift is a change in observed wavelength from relative motion (or expanding space), not light travelling faster or slower.
- Relative source-observer motion changes observed frequency or wavelength.
- Compare a identified line with its laboratory rest wavelength.
- Watch for: the approximate shift relation does not show that light travels faster from an approaching source.
Write from a redshift of 0.005
- A 500 nm rest line is observed at 502.5 nm.
- Find z. Redshift does not mean the speed of light increased.
An observed redshift means light speed increased.
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
Light speed in vacuum is the same for all observers. Redshift is a change in observed wavelength from relative motion (or expanding space), not light travelling faster or slower.
Shift of an identified line toward longer wavelength: write the technical term.
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redshift means Shift of an identified line toward longer wavelength.
A 500 nm rest line is observed at 502.5 nm. What is z, and why does a redshift not mean the speed of light increased?
The fractional shift is the change in wavelength divided by the rest wavelength.
What is z?
Сложность 1/5 · 100 монет + небольшой бонус за первое выполнение сегодня
2.5 divided by 500 is 0.005. Other options — 0.002 (the lesson shift from 600 nm to 601.2 nm); 2.5 nm (the wavelength change).