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Spectra and Energy Levels in Atoms
The light emitted by a filament lamp is white light.  
When white light is passed through a prism or diffraction grating, a spectrum can be observed.  
The spectrum of white light includes all the colours, as shown here and is called a continuous spectrum.  
   
 
   
However, if an electric current is passed through a low pressure gas, only certain colours (wavelengths) of light are emitted.  
We say that this type of light source gives a line spectrum, as shown in the next diagram.
   
 
   
This diagram represents (approximately) the spectrum obtained when the gas consists of hydrogen atoms.  
   
Johann Balmer measured the wavelengths of visible light emitted by hydrogen atoms.  
He found that they could be described by the following formula (now called the Balmer series).  
   
 
where l is the wavelength, R is a constant (the Rydberg constant) and n = 3, 4, 5, etc.  
   
In order to explain the Balmer series, Bohr made the following suggestions  
   
1. Electrons in atoms can only have certain allowed energies.
   
2. An electron can be excited from one energy level to another (higher level) by a collision with another particle or by absorbing a quantum of electro-magnetic radiation.
   
3. When an electron falls from one energy level to a lower level, it emits one quantum of electro-magnetic radiation.
   
4. The energy possessed by the quantum, is the difference between the initial and final energy levels of the electron.
 
 
   
Energy transitions in atoms are often represented by diagrams like the one below.  
The lowest energy is called the "ground state".  
   
 
 
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