Slide 1
An excited gas emits selected spectral lines
A thin excited gas can emit a pattern of narrow spectral lines. Each line corresponds to an allowed energy change in its atoms or ions.
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A dense hot solid usually produces a much broader spectrum because interactions among many particles alter the available states. The appearance of a spectrum depends on the material’s physical environment as well as its composition.
Slide 2
Atomic energy differences set photon frequencies
When an atom loses energy by emitting light, a photon, a packet of light energy, carries the difference. Larger energy differences correspond to higher photon frequencies.
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The relation is photon energy equals Planck’s constant times frequency. Because only particular bound-state energies are available, the possible transitions give a structured set of frequencies rather than an arbitrary continuum.
Slide 3
Atoms can absorb matching wavelengths too
Atoms can absorb photons that match allowed transitions from occupied lower levels. A spectrum may therefore contain dark absorption lines.
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Whether a particular line is strong depends on how many atoms occupy the relevant starting state and on the transition probability. A missing strong-looking line is not explained by abundance alone.
Slide 4
Atoms and their ions need different reference patterns
Different elements have different energy structures. Removing electrons changes the spectrum too, so an atom and its ions need separate reference patterns.
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Laboratories compile measured wavelengths and transition data for comparison. Identification is stronger when several expected lines agree than when one isolated feature happens to lie near a reference wavelength.
Slide 5
Motion, collisions and instruments broaden real lines
Spectral lines are not infinitely thin in a measurement. Particle motion, collisions and instrument resolution can all broaden their appearance.
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The line position and shape carry additional information about conditions. Interpreting them requires separating effects from the source and from the instrument, which is why spectrometers need wavelength and resolution calibration.