Kinds of Spectra
Hot solids glow in all colours; gases glow in a few; and missing colours tell what light passed through.
Lesson at a glanceShow the key points
- A spectrum: dispersed light displaying its wavelength composition.
- Pure spectrum (no overlap of wavelengths) needs a parallel beam from a narrow slit and focusing of each wavelength to one place; a wide slit gives an overlapping, impure spectrum.
- Emission spectra: continuous (bulb, candle, hot iron), line (excited atoms; e.g. sodium 589.0 and 589.6 nm), band (molecules; grouped levels).
- Absorption spectra: dark lines (atoms; Fraunhofer lines in sunlight) or dark bands (molecules) on a continuous background.
- Ultraviolet ≈ 15–380 nm (studied with quartz prisms, photographic plates); infrared ≈ 780–40,000 nm (rock-salt prisms, thermopile detects its heating). Beyond: X-rays, γ-rays; radio waves.
Spread the light from a source into its colours and you get a barcode that identifies what made it. Astronomers know what distant stars are made of because of this.
Pure and impure spectra
Ideally each wavelength lands in its own place, with no overlapping: a pure spectrum, with sharp, distinct colours. This needs:
- a parallel beam falling on the prism, from a narrow slit placed at the focus of an achromatic lens; and
- the dispersed light to be focused by a second achromatic lens so all rays of one wavelength meet at one point.
With a wide slit, each part of the slit makes its own spectrum, slightly shifted, and they overlap: an impure spectrum with smeared colours.
Emission spectra
A source excited by heating or an electric discharge emits light; its dispersed light is an emission spectrum, of three kinds.
Continuous spectrum
A filament lamp, a candle or red-hot iron emits all wavelengths over a range. The colours merge smoothly with no gaps or sharp boundaries.
Line spectrum
Atoms can only have certain energies. An excited atom drops to a lower energy by emitting light of particular wavelengths only. A gas of such atoms gives a few sharp bright lines on a dark background, a fingerprint of the element. Sodium vapour, for example, gives two yellow lines at 589.0 and 589.6 nm.
Band spectrum
Molecules have energy levels grouped in bunches: close together within a bunch, widely spaced between bunches. Their emitted wavelengths are grouped too, and each group looks like a continuous band.
Absorption spectra
Send white light through a material that absorbs certain wavelengths. The dispersed transmitted light shows dark lines or bands where those wavelengths are missing, on an otherwise continuous bright background.
Atoms absorb a few separate wavelengths: sharp dark lines. Sunlight shows many: the Fraunhofer lines.
Molecules absorb groups of wavelengths: dark bands, as with light through some solutions or gases.
Beyond the visible: ultraviolet and infrared
Excited objects can emit outside the visible range of about 380–780 nm. We usually say “light” for the visible and “radiation” in general:
- Ultraviolet: about 15–380 nm. Glass absorbs it, so quartz prisms and photographic plates are used to study it.
- Infrared: about 780–40,000 nm. Rock-salt prisms are used; infrared heats what it falls on, so a heat-sensitive thermopile detects it.
- Beyond ultraviolet come X-rays and gamma rays; beyond infrared, radio waves.
- 1Pure spectrum: parallel light from a narrow slit, each wavelength focused to one place; a wide slit gives an impure spectrum.
- 2Emission: continuous (hot solids), line (atoms), band (molecules).
- 3Absorption: dark lines (atoms; Fraunhofer lines in sunlight) or dark bands (molecules).
- 4Ultraviolet ≈ 15–380 nm (quartz), infrared ≈ 780–40,000 nm (rock salt, thermopile).
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