Spectrum
Light spread out by wavelength (colour). Atoms and molecules each emit and absorb only certain wavelengths, so thin lines appear in the spectrum.
In 1672 Isaac Newton split sunlight into a rainbow band with a prism and showed that white light is a mixture of lights of many colours[1]. That band of colours is a spectrum.
In 1860 Gustav Kirchhoff and Robert Bunsen confirmed that each heated element gives off bright lines at its own fixed positions, establishing how to identify the elements in a substance from its spectral lines[2]. The method uncovered new elements and made it possible to study what distant stars are made of.
Why each element should have its own lines remained a mystery for decades. When Bohr's 1913 model explained the lines as jumps in an electron's energy, the spectrum became a window into the inside of the atom.
Related terms
- Balmer formulaA simple whole-number formula giving the wavelengths of the visible lines in hydrogen's light, found in 1885 by the Swiss schoolteacher Johann Balmer.
- Blackbody radiationLight emitted by an ideal body that absorbs all light falling on it. Its spectrum depends only on temperature, not on what the body is made of.
- Bohr modelThe 1913 model in which electrons circle only on certain orbits and emit light of definite energy when jumping between them.