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Light Emission
www.physics.sfasu.edu/friedfeld/ch29lec.ppt‎
Radio waves are produced by
electrons moving up and down
an antenna.
Visible light is produced by
electrons changing energy
states in an atom.
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.”
Flame Colors
Emission Spectra
Continuous Emission Spectrum
Slit
White Light
Source
Prism
Photographic Film
Emission Spectrum
of Hydrogen
Discrete Emission Spectrum
Slit
Film
Low
Density
Glowing
Hydrogen
Gas
Prism
Photographic Film
Spectra of the Noble Gases
INCANDESCENCE
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.
Peak Frequency
Frequency is
proportional to
temperature.
f T
Relative Energy
Brightness versus Color curve for different
temperatures
Frequency
(measured in Kelvins)
Absorption Spectrum
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.
FLUORESCENCE
Some materials that are excited by UV emit
visible.
These materials are referred to as fluorescent
materials.
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.
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.
Bioluminescence: Created by chemical
reactions inside living organisms.
LASERS
Light
Amplification by
Stimulated
Emission of
Radiation
Lasers produce coherent light.
Coherent light is light with the same
frequency and the same phase.
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