Photons, Electrons, and
Atoms
Visible and non-visable light
• Frequencies around 1015 Hz
• Much higher than electric circuits
• Theory was about vibrating charge in the
atom
• This lead to the “quantum” theory of light
• It states that some of the properties of EM
radiation resemble those of particles
• Where KE is proportional to f
The Photoelectric Effect
• It is caused by an element absorbing
certain wavelength of light which causes
electrons to be ejected from the surface
• This “potential energy barrier” can be
found with the work function f
• Classic mechanics predicted that the
amplitude of the wave, not its frequency,
would cause more electrons to be emitted
Photoelectric cont.
• It was found that the amplitude, no matter
how bright, would not cause the electrons
to leave.
• Only when critical or threshold frequency
was reached, would the electrons move
• For most metals it’s in the ultraviolet range
• At frequencies higher than ft, the stopping
potential would completely stop the flow of
electrons from the surface
Equations
• Stopping potential
1
2
m vmax  eV0
2
• Energy of a photon
E  hf
• Planck’s constant h = 6.6260755 x10-34 j.s
1
2
m vmax  hf  f  eV0
2
The eV
• The electron volts relates electrons and
energy
• 1 eV = 1.602 x 10-19 J
• Ex: If Vo = 5V then eV = 5eV
• Example 41-1
hc
• Photon energy E  hf 

• p = momentum
E  pc  p 
h

EM Spectrum
Line spectra
• Line spectra phenomenon vs spectrum
• Each line is an image of the spectrograph
slit, deviated at an angle that depends on
the frequency of the light
• Each element emits a “signature”
• Certain wavelengths of light unique to its
atomic structure
• Classical mechanics could not explain this
Energy
• Bohr proposed the idea of energy levels
• All atoms of an element have the same
energy levels, with no electrons existing in
“intermediate” levels
• The electrons can “jump” from a higher to
a lower level by emitting a photon
• hf = Ei - Ef
Balmer Series
• Trial and error
resulted in the
formula
• R = 1.097 x 107 m-1
• Substituting in for 
we get En ( energy
levels
1 1 
 R 2  2 

2 n 
1
hcR
En   2
n
Emission spectra
Sources
• http://cwx.prenhall.com/bookbind/pubbook
s/hillchem3/medialib/media_portfolio/07.ht
ml