When white light is passed through cool gases, the spectra observed are called

Question

BlackTom AI · Accepted Answer

Atoms in a cool gas absorb photons at specific energies, producing dark lines (absorption lines) superimposed on an otherwise continuous spectrum.

Relevant knowledge points and concepts:
- Continuous spectrum: produced by a hot, dense source (like a blackbody or incandescent filament) that emits all visible wavelengths smoothly.
- Emission line spectrum: produced by a hot, low-density gas whose atoms emit photons at discrete wavelengths, appearing as bright lines on a dark background.
- Absorption line spectrum: occurs when continuous white light passes through a cooler, low-density gas; atoms in the gas absorb photons at their characteristic energies, removing those wavelengths and leaving dark lines (Fraunhofer lines) in the transmitted light.

Step-by-step reasoning:
1. Start with white light: it contains a continuous range of wavelengths (continuous spectrum).
2. The white light travels through a cool gas. Atoms in the gas have electrons in lower energy states.
3. Photons whose energies match allowed atomic transitions are absorbed, promoting electrons to higher energy levels.
4. Those absorbed wavelengths are missing from the transmitted light; when the transmitted light is dispersed (e.g., with a prism), the spectrum shows dark (absorption) lines at the absorbed wavelengths.

No numeric calculations are required for this conceptual process; the intermediate logical steps are listed above.

Why the correct answer is correct:
- Option b (absorption line spectra) precisely describes dark lines superimposed on a continuous background produced when white light passes through a cooler gas.

Why the other options are incorrect:
- Option a (continuous spectra): incorrect because the presence of the cool gas produces discrete missing wavelengths, not a smooth continuum.
- Option c (emission line spectra): incorrect because that describes bright lines from a hot, excited gas emitting on a dark background, not dark lines on a continuous spectrum.
- Option d (line spectra): too vague; it does not specify whether lines are absorption or emission. The specific phenomenon here is absorption lines.

Conclusion: The correct answer is b. absorption line spectra, which matches the provided answer.

When white light is passed through cool gases, the spectra observed are called单项选择题

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