Download Chapter 29

Chapter 30
Light Emission
Radio waves are produced by
electrons moving up and down
an antenna.
Visible light is produced by
electrons changing energy
states in an atom.
1. EXCITATION
Excitation - occurs when an electron
in an atom is given energy causing it to
jump to a higher orbit.
This can happen through
collisions or
photon absorption
(the photon absorption must exactly
match the energy jump).
Here comes a nucleus
With possible orbits for electrons
e-
In this configuration the atom is not excited.
Here comes a photon
e-
In this configuration the atom is not excited.
e-
Now the atom is said to be excited because
the electron is in a higher than normal orbit.
e-
Now the atom is in a non-excited state again.
It emitted a photon when it changed orbits.
The excited atom usually de-excites in
about 100 millionth of a second.
The subsequent emitted radiation has an
energy that matches that of the orbital
change in the atom.
Atomic Excitation
This emitted radiation gives the characteristic
colors of the element involved.
The atoms do not “wear out.”
Demo - Flame Colors
Classical explanation fails - it says there
should be radiation even when there has
been no change in energy levels.
The electron should spiral into the
nucleus - the ultraviolet catastrophe.
E = hf
(h = Planck’s constant)
Emission Spectra
Continuous Emission Spectrum
Slit
White Light
Source
Prism
Photographic Film
Emission Spectra of
Hydrogen
Discrete Emission Spectrum
Slit
Film
Low Density
Glowing
Hydrogen Gas
Prism
Photographic Film
Spectra of Some Gases – Next Slide
Demo - Spectra of a Discharge Tube Gas
and Sodium Vapor Lamp
2. INCANDESCENCE
Demo - Spectrum of Incandescent Bulb
Blue hot is hotter than white hot which is hotter
than red hot.
White light - all colors in the visible are present.
Electron transitions occur not only in the parent
atom but in adjacent atoms as well.
Relative Energy
Brightness versus Color curve for different
temperatures
Peak Frequency
Frequency
f T
(measured in Kelvins)
Absorption Spectra
of Hydrogen
Discrete Emission Spectrum
Discrete Absorption Spectrum
Slit
Hydrogen Gas
Film
White Light
Source
Prism
Photographic Film
Absorption Spectra
Frequencies of light that represent the correct energy
jumps in the atom will be absorbed.
When the atom de-excites, it emits the same kinds of
frequencies it absorbed.
However, this emission can be in any direction.
Close inspection of the absorption spectrum of
the sun reveals missing lines known as
Fraunhofer lines.
In 1868 a pattern of lines was observed in the
solar spectrum that represented an element that
had not been found on earth. It was
Helium - named for Helios, the sun.
Doppler shifts are observed in the spectra of
stars.
If a star is approaching, its spectra will be
blue shifted.
If a star is moving away, its spectra will be
red shifted.
Most spectra are red shifted
indicating that on the average the
universe is expanding.
3. FLUORESCENCE
Some materials that are excited by UV emit
visible.
These materials are referred to as fluorescent
materials.
Demo – Fluorescence with Green Laser and
Liquids
Fluorescent Lamps
Primary excitation - electron collisions with low
pressure Hg vapor, UV given off
Secondary excitation - UV photons absorbed by
phosphors. Phosphors fluoresce emitting visible
light.
Remember that the visible light from the excited
mercury vapor is also emitted.
4. PHOSPHORESCENCE
Electrons get "stuck" in excited states in the
atoms and de-excitation occurs at different
times for different atoms.
A continuous glow occurs for some time.
Demo - Fluorescent & Phosphorescent
Objects (including helicopter and
“Starry, Starry Night”)
Bioluminescence
5. LA
LASERS
SER
Light
Amplification by
Stimulated
Emission of
Radiation
Lasers produce coherent light.
Coherent light is light with the same
frequency and the same phase.
Demo - Making Laser Beam Visible
Early model - ruby crystal doped with Cr
Slide - Helium-Neon Laser
Lasers
Chapter 30 Review Questions
What type of spectrum would you expect
to obtain if white light is shined through
sodium vapor?
(a) an emission spectrum of sodium
(b) an absorption spectrum of sodium
(c) a continuous spectrum
In fluorescence which has the higher
energy, the radiation absorbed or the
radiation of an emitted photon?
(a) absorbed
(b) emitted
(c) they both have the same energy
Which phenomenon has electrons
getting "stuck" in excited states?
(a) incandescence
(b) fluorescence
(c) phosphorescence
What causes laser light to have all of its
waves moving in the same direction?
(a) the mirrors in the laser
(b) the stimulated emission of the atoms to
radiate in the same direction
(c) atoms are lined-up in the crystal so that
they emit light only in one direction
Link to Chapter 27