ACT Science Practice Question #1273 (Medium (31)) | Test Citadel
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ACT Science Difficulty: Medium (31)

Digital ACT Science Practice Question #1273

Test your problem-solving accuracy on this official-caliber item. Select an answer choice to check your reasoning instantly.

An astrophysicist measures the hydrogen absorption spectrum from Star Alpha. The rest laboratory wavelength for the designated spectral line is $\lambda_0 = 656.3 nm$, while the observed wavelength in the stellar spectrum is $\lambda_{\text{obs}} = 654.1 nm$. Based on the Doppler formula $z = \frac{\lambda_{\text{obs}} - \lambda_0}{\lambda_0}$, how should this spectral line be categorized, and what does it indicate regarding the object's line-of-sight motion relative to Earth?
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Socratic AI Engine Step-by-Step Derivation
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Shorter observed wavelength = Blue = Moving toward us. Longer observed wavelength = Red = Moving away from us.

Masterclass Solution & Distractor Trap Analysis

Compare observed wavelength $\lambda_{\text{obs}} = 654.1 nm$ with rest wavelength $\lambda_0 = 656.3 nm$: $\lambda_{\text{obs}} < \lambda_0$, meaning the wavelength has compressed (shifted toward the blue end of the spectrum). A blueshift ...

Distractor Analysis: Trap choice eliminates careless test-takers who confuse roots with coordinates...

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