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. 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 these 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 wavelengths. Each element has an individual, characteristic line spectrum, called then an 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 the line spectrum for chemistry.
Light emitted by gas is of course really emitted by its individual atoms. But why do atoms of a particular element emit light of only some frequencies? Scientists were almost sure that the phenomenon dealt with electrons of the atom. But they couldn't say how the spectrum depended on electrons. The radiation transfers out some of the energy of the electron. So the electron loosing its energy should move on smaller and smaller orbits and finally fall on the nucleus. But no such phenomenon occurs. If it was so then an electron could be placed on any orbit (which would be fluently changing with the energy emission). According to all that atoms should be in different energy states and emit the radiation of all wavelengths. The spectrum should be continuous one not the line one.
All those divergences led Niels Bohr, the great physicists of the beginning of the 20th century, to a new theory describing the laws governing the atom.
The simplest one is the atom of hydrogen. There is only one electron in it. The line spectrum of hydrogen is also the simplest one. Scientists managed to determine a formula defining the connection between consecutive spectral lines of that element. Bohr began then to study the features of the atom of hydrogen. Just like Rutherford he assumed that electrons rotate/rotate around the nucleus on round orbits. But he had three completely new ideas:
1. There are some orbits called by him the stationery ones, where the moving electrons don't emit energy.
2. Each emission or absorption of radiation represents the electron transition from one stationery orbit to another. The radiation emitted during such transition is homogeneous and its frequency is given by the formula:
Where h is Planck's constant, En and El are the energies in the two stationary states.
3. The laws of mechanics describe the dynamic equilibrium of electrons in stationery states but do not describe the situation of the electron transition from one stationery orbit to another.
Let's now think what each postulate means.
The first one says that electrons can't move on any orbit around the nucleus. Only some orbits are permissible. Electrons moving on them don't loose energy for radiation. The postulate was in complete disagreement with other theories, and especially with the Maxwell theory of electromagnetism. Bohr formulated the postulate ad hoc. He didn't know what it might come from. But he was of the opinion that to properly understand the nature of the atom one has to accept his idea.
The second postulate says that in an atom electrons can change orbits. On each orbit the electron has some defined energy. The energy of the electron is different on different orbits. The bigger the orbit, the bigger the energy. If the electron changes a higher orbit into a lower one then it emits a quantum of energy that is the same as the difference of energy of the higher and lower orbits. To change a lower orbit into a higher one the electron has to absorb an adequate quantum of energy. The quantum of energy is proportional to the frequency of the emitted radiation. The second postulate explains why the atom emits radiation of strictly defined wavelengths.
The third postulate is in a complete disagreement with the classical theory. According to that postulate the laws of mechanics can only describe electrons moving on stationary orbits and not while changing their orbits.
Using his theory Bohr managed to calculate the radiuses of the stationery orbits of the hydrogen atom (and of the atoms similar to hydrogen, which means having only one electron circulating/rotating). He calculated the energy of an electron on consecutive orbits. So he could find theoretically the wavelengths of light emitted by hydrogen. Unfortunately the theory didn't describe the spectrums of more complicated atoms. Moreover the postulates had almost no foundation. Nobody knew what they came of. But an explanation came soon.
The red light has the longest wavelength and the smallest frequency, and the violet light has the shortest wavelength and the biggest frequency.
Each element emits individual, characteristic line spectrum, thanks to which the element can be identified.
Niels Bohr explained the emissive spectrum of the hydrogen atom.
There are some orbits in the atom, where the moving electrons don't emit energy - these are the stationery orbits.
To change a lower orbit into a higher one the electron has to absorb a photon of the proper energy.
To change a higher orbit into a lower one the electron has to emit a photon of the proper energy.
The laws of the classical mechanics don't describe the electron transition from one stationery orbit to another.
Why are the Bohr postulates correct?