chapter 5 microscopic interactions Flashcards
Whenever electromagnetic radiation encounters matter one of three things can happen.
reflection
transmission
absorption
reflection
transmission
absorption
Reflection:
All electromagnetic reflections are governed by the same
physical laws as reflections of visible light
Optics describes
describes the general laws of reflection
Optics describes the general laws of reflection and may be applied to
Øall types of electromagnetic reflections ranging from radio waves to gamma rays.
Transmission:
The electromagnetic radiation may be transmitted
•completely through the substance it encounters.
If absolutely no energy is absorbed by the material, it is said to be
•transparent to the radiation.
The velocity of the radiation is usually ………………… in the transparent medium
slower
The velocity of the radiation is usually slower in the transparent medium and as a result the radiation usually
undergoes refraction
Various materials are transparent at various wavelengths. For example, lead glass is transparent to …………… but not …………………
visible light but not X-rays
several thicknesses of black paper sheets are transparent to …………………… but not …………….
•X-rays, but not visible light.
……………………………. is perfectly transparent.
No known material
Absorption:
The electromagnetic radiation may be
•totally or partially absorbed by the substance.
The electromagnetic radiation may be totally or partially absorbed by the substance.
In this process energy is
transferred to the absorbing medium
In this process energy is transferred to the absorbing medium and this may cause
significant changes to occur within the absorbing medium
Electromagnetic radiation
•is a form of energy whose behavior is described by the properties of both waves and particles.
•Some properties of electromagnetic radiation, such as its ………………….. when it passes from one medium to another, are explained best by ……………………
refraction
describing light as a wave.
Other properties, such as absorption and emission, are better described by
light as a particle.
The exact nature of electromagnetic radiation
remains unclear
The exact nature of electromagnetic radiation remains unclear, as it has since the
•development of quantum mechanics in the first quarter of the 20th century.
……………………………..provide a useful description for electromagnetic radiation.
the dual models of wave and particle behavior
Electromagnetic radiation consists of
oscillating electric and magnetic fields
Electromagnetic radiation consists of oscillating electric and magnetic fields that
•that propagate through space along a linear path and with a constant velocity.
In a vacuum electromagnetic radiation travels at
•at the speed of light, c, which is 2.997 92 × 108 m/s.
When electromagnetic radiation moves through a medium other than a vacuum its velocity, v, is
•less than the speed of light in a vacuum.
The oscillations in the electric and magnetic fields are…………………to each other
perpendicular
The oscillations in the electric and magnetic fields are perpendicular to each other, and to the direction of the
wave’s propagation.
A
magnetic field
B
electric field
c
A
D
lamda ,\
E
direction of propagation
An electromagnetic wave is characterized by several fundamental properties
including its velocity, amplitude, frequency, phase angle, polarization and direction of propagation
the amplitude of the oscillating electric field at any point along the propagating wave is
At is
the magnitude of the electric field at time t
Ae
is the electric field’s maximum amplitude
v is
the wave’s frequency - the number of oscillations in the electric field per unit time
o| is
a phase angle which accounts for the fact that At need not have a value of zero at t=0
the identical equation for the magnetic field is
Other properties also are useful for characterizing the wave behavior of
electromagnetic radiation.
The wavelength, λ, is defined as
the distance between successive maxima
For ultraviolet and visible electromagnetic radiation the wavelength is usually expressed in
nanometers (1 nm = 10–9 m)
for infrared radiation it is given in
microns (1 μm = 10–6 m)
The relationship between wavelength and frequency is
Another unit useful unit is the wavenumber, ν , which is
the reciprocal of wavelength
Wavenumbers are
frequently used to characterize infrared radiation
Wavenumbers are frequently used to characterize infrared radiation, with the units given in
cm–1
When matter absorbs electromagnetic radiation it undergoes
a change in energy
The interaction between matter and electromagnetic radiation is easiest to understand if we assume that
radiation consists of a beam of energetic particles called photons.
When a photon is absorbed by a sample it is
“destroyed,” and its energy acquired by the sample.
The energy of a photon, in ……………, is related to its
in joules, is related to its frequency, wavelength, and wavenumber
•The energy of a photon, in joules, is related to its frequency, wavelength, and wavenumber by the following equalities
When a photon is absorbed by an atom or molecule, which undergoes a transition from a
lower-energy state to a higher energy, or excited state
Absorption
emission
The type of transition depends on
the photon’s energy
For example, absorbing a photon of visible light promotes
one of the atom’s or molecule’s valence electrons to a higher-energy level
When an molecule absorbs infrared radiation, on the other hand, one of its chemical bonds experiences
a change in vibrational energy
When we look at the electronic configuration of an atom we can divide up the electrons in the atom into two categories:
valence and core electrons
The valence electrons are those that are
used in chemical bonding
the core electrons are
not involved in bonding
A & B
C
D & E
G & H & I & J
F
K & L & M
If there are no available quantized energy levels matching the quantum energy of the incident radiation, then
•then the material will be transparent to that radiation
Energy levels are everything in
quantum mechanics
A & B
c
D
For a given frequency of radiation, there is
•only one value of quantum energy for the photons of that radiation
Transitions between energy levels occur by
•absorption, emission and stimulated emission of photons
When an atom in an excited state falls to a lower energy level, it
emits a photon of light.
how does the arrow go
Molecules typically remain excited for no longer than
a few nanoseconds
Molecules typically remain excited for no longer than a few nanoseconds. This is often also called
fluorescence
Molecules typically remain excited for no longer than a few nanoseconds. This is often also called fluorescence or, when it takes longer
phosphorescence
A spectral line is
a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies.
A
Continuous spectrum
B
emission lines
C
Absorption lines
Vij =
ΔEij/h
A
allowed transitions
B
positions of the absorption lines in the spectrum of the molecule
Line positions are determined by the
•energy changes of allowed transitions
Line strengths are determined by the
•fraction of molecules that are in a particular initial state required for a transition
Spectral lines are often used to
identify atoms and molecules from their characteristic spectral lines
These “fingerprints” can be compared to the previously collected “fingerprints” of atoms and molecules, and are thus used to identify the
•atomic and molecular components of stars and planets which would otherwise be impossible.
Spectroscopy is
the study of the interaction between matter and electromagnetic radiation
Historically, spectroscopy originated through the study of
•visible light dispersed according to its wavelength, by a prism.
•Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, by a prism.
Later the concept was expanded greatly to include
any interaction with radiative energy as a function of its wavelength or frequency
What is the energy of a photon from the sodium D line at 589 nm?
(law)
•What is the energy of a photon from the sodium D line at 589 nm?
(solve)
