Example 1
Element A emits at 430,520 and 610 nm; B at 450 and 590 nm. A sample has 430,450,520,590 and 610 nm lines. Identify it.
All listed lines match the union of A and B, so the data are consistent with a mixture of A and B.
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Year 12 · SACE Stage 2
Continuous and line spectra
Classify spectra and match line patterns to element identities.
Method and worked examples
Hot solids, liquids and dense gases emit approximately continuous thermal spectra. As temperature rises, the peak shifts toward higher frequency or shorter wavelength and emitted intensity increases. A low-density excited atomic gas instead produces distinct emission lines. Their frequencies depend on that element’s allowed energy differences, so a line pattern can identify atoms.
A cooler gas in front of a continuous source absorbs selected frequencies, producing dark absorption lines. These correspond to allowed upward transitions and can be a subset of the element’s emission lines. A mixture can show lines from several elements. For fixed data questions, compare positions rather than overall brightness: intensity can depend on abundance, temperature and excitation. Spectral classification distinguishes evidence about atomic energy levels from the continuous radiation of an incandescent source.
Example 1
Example 2
A glowing solid changes from dull red to white as it is heated. Classify its spectrum.
It emits a continuous thermal spectrum whose distribution moves toward shorter wavelengths and becomes more intense.
Watch out: Incandescent dense sources produce continuous spectra; isolated excited atoms produce characteristic line spectra.