1. LINE SPECTRA
New methods of splitting white light into its constituent wavelengths in thew 19th century (the diffraction grating) revealed that the spectrum was not completely continuous.
Fraunhoffer discovered dark lines in the spectrum of the Sun

5. Increasing frequency
A single series of lines converges towards the high frequency end of the spectrum

6. BRIGHT LINES AND DARK LINES
It soon became apparent that these lines were associated with individual elements.
When gases were heated or stimulated electrically they shone with characteristic colours.
When there spectra were taken they were not continuous but consisted of a series of bright lines.
These bright line spectra were called
“emmision spectra”

7. Hydrogen gas at low pressure
High
Voltage
Diffraction grating

8. Emission spectra always consist of several series of lines becoming closer or “converging” at higher frequency
Emission spectrum showing a single series of lines
Lines converge towards the blue

9. Absorption Spectra White light source
Diffraction grating
screen
Cool hydrogen gas
When white light is shone throough the cool gas, black lines appear in the continuous spectrum in exactly the same places as the bright lines appeared in the hot gas.

10. White Light
Hydrogen
Helium
Lithium
Mercury
Absorption spectrum of hydrogen

11. Explaining the lines for hydrogen
Hydrogen is the simplest atom. It has only one electron. Here the electron is in its lowest energy state: its “ground state”
More electron shells that the electron can occupy exist which are higher in energy.
The electron cannot occupy a position “in between” these “shells”.
An electron which occupies a higher energy state than the ground state will eventually fall back to the ground state
When the electron does this it will release a photon of fixed energy

12. The diagram shows an electron in
the second energy level.
In falling to the ground state it has to
lose photon of fixed energy
hf =E1-E2
Where E1 is the energy of the
level it falls
from and E2 is the energy of
the finishing level.

13. The electron gets to a higher level by absorbing a photon of exactly the right frequency to give it the energy to get there.

16. Increasing energy