In the 19th century scientists were sure that light is a wave (they didn't know yet that it is also particles). Each wave has some length and frequency. Those quantities are in connection. And the colour of light depends on the wavelength.
The red light has the longest wavelength and violet has the shortest wavelength. Usually we see light consisting of waves of different wavelengths. For example it is so with ordinary white light. It consists of waves of all wavelengths from red to violet. The resultant of all that colours is white. You can check that very easily. All you need to do it is an ordinary prism. It can disperse light coming through it. So you can see the components. Also a rainbow is an effect of white light dispersion on drops of rain.
What happens if dispersing not white light but the light emitted by gases heated to high temperatures? Well, after that dispersion we won't see a continuous spectrum (which means including all colours) but a discontinuous, line spectrum. Light emitted by gases consists then of some number of waves of different wavelength. Each element has individual, characteristic line spectrum, called then emission spectrum. The spectrum can be used for identifying different substances. Thanks to it, it was possible to define the constitution of the sun and other celestial bodies. It was also possible to discover some unknown before elements. It would be hard to overestimate the importance of line spectrum for chemistry.
Hydrogen has the simplest spectrum. The 19th scientist living in the 19th century, Johann Jakob Balmer proved that the visible part of the hydrogen spectrum can be estimated by the formula:
(1)
(2)
The system of the lines was called the Balmer spectral series. For hydrogen there are also some other series:
- The Lyman spectral series - placed in ultraviolet (the wavelengths are shorter than for the visible light) - is described by the given above formula but n' is equal to 1 here, and n is equal to 2 or more.
- The Paschen spectral series - placed in infrared (the wavelengths are longer than for the visible light) It is described by the first formula but n' is equal to 3 here, and n is equal to 4 or more.
- The Brackett spectral series - placed in infrared - is described by the first formula but n' is equal to 4 here, and n is equal to 5 or more.
The spectral lines of other elements, which are heavier than hydrogen, are more complicated.
The question of how the spectral lines arise was answered just at the beginning of the 20th century by Niels Bohr. And the explanation was based on the newly developed theory of quanta.
