Fill in the blanks using the options below.

When an emitter moved TOWARD you, it becomes ___________, and when it moves AWAY from you, it becomes ___________.

Question

Fill in the blanks using the options below.

When an emitter moved TOWARD you, it becomes ___________, and when it moves AWAY from you, it becomes ___________.

BlackTom AI · Accepted Answer

When a light source moves toward an observer its wavelengths are compressed (shorter) and when it moves away they are stretched (longer). This is the electromagnetic Doppler effect: motion along the line of sight changes the observed frequency and wavelength.

1) Relevant knowledge points and concepts
- Doppler effect for light: relative motion between emitter and observer changes observed frequency (f) and wavelength (λ).
- Shorter wavelengths correspond to bluer light (blueshift); longer wavelengths correspond to redder light (redshift).
- For small speeds v (v << c), the fractional change in wavelength is approximately Δλ/λ ≈ ±v/c, with the + sign for recession and − sign for approach.

2) Step-by-step reasoning
- Let λ_em be the emitted wavelength. If the source is moving away at speed v, observed wavelength λ_obs ≈ λ_em(1 + v/c). That increases λ (redder).
- If the source is moving toward us at speed v, λ_obs ≈ λ_em(1 − v/c). That decreases λ (bluer).

3) Intermediate calculation examples
- Example: v = 0.01c. Receding: λ_obs ≈ λ_em(1 + 0.01) = 1.01 λ_em (longer → redshift). Approaching: λ_obs ≈ λ_em(1 − 0.01) = 0.99 λ_em (shorter → blueshift).

4) Why the correct answer is correct
- Approaching motion shortens wavelengths so the spectrum is shifted toward blue wavelengths → blueshifted. Receding motion lengthens wavelengths so the spectrum is shifted toward red wavelengths → redshifted.

5) Why other multiple-choice options are incorrect
- "blushifted / null shifted": misspells "blueshifted" and "null shifted" is not the correct description for recession (would only apply when there is no relative motion).
- "redshifted / blueshifted": reverses the cause–effect (it claims approach causes redshift and recession causes blueshift), so it’s incorrect.
- "null shifted / redshifted": implies approach gives no shift, which is false; approach produces a blueshift.

Conclusion: Therefore, when an emitter moves TOWARD you it becomes blueshifted, and when it moves AWAY from you it becomes redshifted — matching the provided answer (blueshifted / redshifted).

Fill in the blanks using the options below. When an emitter moved TOWARD you, it becomes _, and when it moves AWAY from you, it becomes _.单项选择题

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Fill in the blanks using the options below. When an emitter moved TOWARD you, it becomes ___________, and when it moves AWAY from you, it becomes ___________.单项选择题