2.2.4Space
Continuous and Line Spectra — SQA National 5 Physics Revision Notes & Topical Questions
5 min read 4 parts Lesson 11 of 12
Part 1 of 4Continuous Spectra
A prism or diffraction grating spreads light out into its component colours.
Appearance
- A continuous spectrum is an unbroken band of colour from red through to violet with no gaps.
Source
- Hot, dense objects such as the filament of a lamp or the core of a star produce it.
Dispersion
- A prism bends each colour by a different amount, so white light splits into its separate colours.
Order
- Red light is bent least and violet light is bent most as the light passes through the prism.
Part 2 of 4Line Spectra
Gases made of individual atoms give out or take in only particular wavelengths.
Emission lines
- A heated gas gives out light only at certain wavelengths, seen as bright coloured lines on a dark background.
Absorption lines
- Cool gas in front of a hot source takes out the same wavelengths, leaving dark lines in a continuous spectrum.
Why lines appear
- Atoms of each element can only gain or lose specific amounts of energy, so only specific wavelengths are involved.
Fingerprint
- Every element produces its own pattern of lines, which acts like a fingerprint.
Link to energy
- When an electron in an excited atom drops to a lower energy level, it gives out light of one definite wavelength.
| Spectrum | Appearance | Typical source |
|---|---|---|
| Continuous | Unbroken band of colour | Hot filament lamp |
| Emission line | Bright lines on dark background | Heated gas such as a discharge tube |
| Absorption line | Dark lines on a bright band | Light passing through cooler gas |
Part 3 of 4Identifying an Element
Matching line patterns lets scientists name elements without ever touching the source.
1
Collect light from the source and pass it through a prism or grating to spread out its spectrum.
2
Record the positions of the lines, noting the colour or wavelength of each one.
3
Compare the pattern with reference spectra of known elements measured in a laboratory.
4
An element is present if all of its characteristic lines appear in the unknown spectrum.
5
If a line is unmatched, another element must also be present in the source.
Part 4 of 4Exam Checklist & Tips
2.2.4 – Key Points for the Exam
- Continuous spectrum An unbroken band of colours containing all wavelengths of visible light.
- Line spectrum A spectrum made of separate lines at wavelengths characteristic of an element.
- Emission Light given out by a source such as a heated gas.
- Absorption Light taken in by a gas, leaving dark lines in a spectrum.
Common Mistakes
- Describing a line spectrum as a continuous band with a few extra colours.
- Mixing up emission lines (bright) with absorption lines (dark).
- Saying a prism adds colours to light; it only separates the colours already there.
- Assuming one matching line is enough to identify an element when its full pattern should match.
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