# General Properties of Electromagnetic Radiation - PowerPoint PPT Presentation The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
Title:

## General Properties of Electromagnetic Radiation

Description:

### General Properties of Electromagnetic Radiation * * The decrease in radiation velocity upon propagation in transparent media is attributed to periodic polarization of ... – PowerPoint PPT presentation

Number of Views:43
Avg rating:3.0/5.0
Slides: 63
Provided by: abo3ali
Category:
Tags:
Transcript and Presenter's Notes

Title: General Properties of Electromagnetic Radiation

1
2
• The electromagnetic radiation is looked at as
sinusoidal waves which are composed of a
combination of two fields. An electric field
(which we will use, in this course, to explain
absorption and emission of radiation by analytes)
and a magnetic field at right angle to the
electric field (which will be used to explain
phenomena like nuclear magnetic resonance in the
course of special topics in analytical chemistry
offered to Chemistry students only).

3
The classical wave model
• The classical wave model describes
electromagnetic radiation as waves that have a
wavelength, frequency, velocity, and amplitude.
These properties of electromagnetic radiation can
explain classical characteristics of
refraction, diffraction, interference, etc.
However, the wave model can not explain the
phenomena of absorption and emission of radiation.

4
• We will only deal with the electric field of the
electromagnetic radiation and will thus refer to
an electromagnetic wave as an electric field
having the shape of a sinusoidal wave. The arrows
in the figure below represent few electric
vectors while the yellow solid sinusoidal wave is
the magnetic field associated with the electric
field of the wave.

5
6
Wave Parameters
• 1. Wavelength (?)
• The wavelength of a wave is the distance between
two consecutive maxima or two consecutive minima
on the wave. It can also be defined as the
distance between two equivalent points on two
successive maxima or minima. This can be seen on
the figure below

7
(No Transcript)
8
• 2. Amplitude (A)
• The amplitude of the wave is represented by the
length of the electrical vector at a maximum or
minimum in the wave. In the figure above, the
amplitude is the length of any of the vertical
arrows perpendicular to the direction of
propagation of the wave.

9
• 3. Frequency

The frequency of the wave is directly
proportional to the energy of the wave and is
defined as the number of wavelengths passing a
fixed point in space in one second.
4. Period (p) The period of the wave is the time
in seconds required for one wavelength to pass a
fixed point in space.
10
• 5. Velocity (v)
• The velocity of a wave is defined as the
multiplication of the frequency times the
wavelength. This means
• V ????
• The velocity of light in vacuum is greater than
its velocity in any other medium

11
• Since the frequency of the wave is a constant and
is a property of the source, the decrease in
velocity of electromagnetic radiation in media
other than vacuum should thus be attributed to a
decrease in the wavelength of radiation upon
passage through that medium.

12
(No Transcript)
13
• 6. Wavenumber (?)
• The reciprocal of wavelength in centimeters is
called the wavenumber. This is an important
property especially in the study of infrared
spectroscopy.
• wavenumber is directly proportional to frequency
and thus E
• k ?
• k depends on medium and 1/velocity

14
Electromagnetic Spectrum
• The electromagnetic radiation covers a vast
spectrum of frequencies and wavelengths. This
includes the very energetic gamma-rays radiation
with a wavelength range from 0.005 1.4 Ao to
radio waves in the wavelength range up to meters
(exceedingly low energy). However, the region of
interest to us in this course is rather a very
limited range from 180-780 nm. This limited range
covers both ultraviolet and visible radiation.

15
(No Transcript)
16
Mathematical Description of a Wave
• A sine wave can be mathematically represented by
the equation
• Y A sin (?t ?)
• Where y is the electric vector at time t, A is
the amplitude of the wave, ? is the angular
frequency, and ? is the phase angle of the wave.
• The angular frequency is related to the frequency
• ? 2??
• This makes the wave equation become
• Y A sin (2??t ?)

17
Mathematic Description of a Wave
w angular frequency 2pn
Y A sin(2??t f), n is the regular
frequency
Douglas A. Skoog, et al. Principles of
Instrumental Analysis, Thomson, 2007
18
Superposition of Waves
• When two or more waves traverse the same space, a
resultant wave, which is the sum of all waves,
results. Where the resultant wave can be written
as
• Y A1 sin (2??1t ?1) A2 sin (2???t ??)
........ An sin (2??nt ?n)

19
Constructive Interference
• The resultant wave would has a greater amplitude
than any of the individual waves which, in this
case, is referred to as constructive
interference. The opposite could also take place
where lower amplitude is obtained.

20
Constructive Interference
21
• The decrease in the intensity is a result of what
is called a destructive interference. When the
multiple waves have the same wavelength, maximum
constructive interference takes place when ?1 -
?2 is equal to zero, 360 deg or multiple of 360
deg.
• Also maximum destructive interference is observed
when ?1 ?2 is equal to 180 deg, or 180 deg
multiples of 360 deg.
• A 100 constructive interference can be seen for
interference of yellow and blue shaded waves
resulting in a wave of greater amplitude, brown

22
The blue and yellow shaded waves interfere to
give the brown shaded wave of less amplitude, a
consequence of destructive interference of the
two waves.
23
The Period of a Beat
• When two waves of the same amplitude but
different frequencies interfere, the resulting
wave exhibit a periodicity and is referred to as
beat (see figure below). The period of the beat
can be defined as the reciprocal of the frequency
difference between the two waves
• Pb 1/(??)

24
(No Transcript)
25
Fourier Transform
• The resultant wave of multiple waves of different
amplitudes and frequencies can be resolved back
to its component waves by a mathematical process
called Fourier transformation. This mathematical
technique is the basis of several instrumental
techniques like Fourier transform infrared,
Fourier transform nuclear magnetic resonance, etc.

26
• Diffraction is a characteristic of
process by which a parallel beam of radiation is
bent when passing through a narrow opening or a
demonstrate its wave nature. Diffraction is not
clear when the opening is large.

27

28
(No Transcript)
29
Diffraction Pattern From Multiple Slits
Douglas A. Skoog, et al. Principles of
Instrumental Analysis, Thomson, 2007
30
Diffraction Pattern From Multiple Slits
Douglas A. Skoog, et al. Principles of
Instrumental Analysis, Thomson, 2007
31
Diffraction Pattern From Multiple Slits
CF BC sin ? n? n is an integer called order
of interference
Douglas A. Skoog, et al. Principles of
Instrumental Analysis, Thomson, 2007
32
patterns
• Two beams of radiation are said to be coherent if
they satisfy the following conditions
• 1. Both have the same frequency and wavelength or
set of frequencies and wavelength.
• 2. Both have the same phase relationships with
time.
• 3. Both are continuous.

33
• As mentioned before, the velocity of radiation in
any medium is less than that in vacuum. The
velocity of radiation is therefore a function of
the refractive index of the medium in which it
propagates. The velocity of radiation in any
medium can be related to the speed of radiation
in vacuum ( c ) by the relation
• ni c/vi
• Where, vi is the velocity of radiation in the
medium I, and ni is the refractive index of
medium i.

34
• The decrease in radiation velocity upon
propagation in transparent media is attributed to
periodic polarization of atomic and molecular
species making up the medium. By polarization we
simply mean temporary induced deformation of the
electronic clouds of atoms and molecules as a
result of interaction with electric field of the
waves.

35
• If we look carefully at the equation ni c/vi
and remember that the speed of radiation in
vacuum is constant and independent on wavelength,
and since the velocity of radiation in medium I
is dependent on wavelength, therefore the
refractive index of a substance should be
dependent on wavelength. The variation of the
refractive index with wavelength is called
dispersion.

36
• When a beam of radiation hits the interface
between two transparent media that have different
refractive indices, the beam suffers an abrupt
change in direction or refraction. The degree of
refraction is quantitatively shown by Snell's law
where
• n1 sin ?1 n2 sin ?2

37
(No Transcript)
38
• An incident beam hitting transparent surfaces (at
right angles) with a different refractive index
will suffer successive reflections. This means
that the intensity of emerging beam will always
be less than the incident beam.

39
(No Transcript)
40
• When a beam of radiation hits a particle,
molecule, or aggregates of particles or
molecules, scattering occurs. The intensity of
scattered radiation is directly proportional to
particle size, concentration, the square of the
polarizability of the molecule, as well as the
fourth power of the frequency of incident beam.
Scattered radiation can be divided into three
categories

41
The fraction of radiation transmitted at all
angles from its original path
• Rayleigh scattering
• Molecules or aggregates of molecules smaller than
?
• Scattering by big molecules
• Used for measuring particle size
• Raman Scattering
• Involves quantized frequency changes

42
• All the previously mentioned properties of
radiation agrees with the wave model of
radiation. However, some processes of interest to
us, especially in this course, can not be
explained using the mentioned wave properties of
radiation. An example would be the absorption and
emission of radiation by atomic and molecular
species. Also, other phenomena could not be
explained by the wave model and necessitated the
suggestion that radiation have a particle nature.
The familiar experiment by Heinrich Hertz in 1887
is the corner stone of the particle nature of
radiation and is called the photoelectric effect.

43
The Photoelectric Effect
• When Millikan used an experimental setup like the
one shown below to study the photoelectric
effect, he observed that although the voltage
difference between the cathode and the anode was
insufficient to force a spark between the two
electrodes, a spark occurs readily when the
surface of the cathode was illuminated with
light. Look carefully at the experimental setup

44
(No Transcript)
45
• It is noteworthy to observe the following points
• 1. The cathode was connected to the positive
terminal of the variable voltage source, where it
is more difficult to release electrons from
cathode surface.
• 2. The anode was connected to the negative
terminal of the voltage source which makes it
more difficult for the electron to collide with
the anode for the current to pass.
• 3. The negative voltage was adjusted at a value
insufficient for current to flow. The negative
voltage at which the photocurrent is zero is
called the stopping voltage.

46
• At these conditions, no current flows through the
circuit as no electrons are capable of completing
the circuit by transfer from cathode to anode.
However, upon illumination of the cathode by
radiation of suitable frequency and intensity, an
instantaneous flow of current takes place. If we
look carefully at this phenomenon and try to
explain it using the wave model of radiation, it
would be obvious that none of the wave
characteristics (reflection, refraction,
interference, diffraction, polarization, etc. )
can be responsible for this type of behavior.

47
• What actually happened during illumination is
that radiation offered enough energy for
electrons to overcome binding energy and thus be
electrons enough kinetic energy to transfer to
the anode surface and overcome repulsion forces
with the negative anode.
• If the energy of the incident beam was calculated
per surface area of an electron, this energy is
infinitesimally small to be able to release
electrons rather than giving electrons enough
kinetic energy. When this experiment was repeated
using different frequencies and cathode coatings
the following observations were collected

48
Conclusions
• 1. The photocurrent is directly proportional to
• 2. The magnitude of the stopping voltage depends
on both chemical composition of cathode surface
• 3. The magnitude of the stopping voltage is
independent on the intensity of incident

49
Energy States of Chemical Species
• The postulates of quantum theory as introduced by
Max Planck in 1900 (E h?)
• Heated objects (or Excitation) Emission of
relaxation .
• Explanation
• Atoms, ions, and molecules can exist in certain
discrete energy states only.
• When these species absorb or emit energy exactly
equal to energy difference between two states
they transfer to the new state. Only certain
energy states are allowed (energy is quantized).

50
• 2. The energy required for an atom, ion, or a
molecule to transfer from a one energy state to
another is related to the frequency of radiation
absorbed or emitted by the relation
• ?E Efinal Einitial hn
• Therefore, we can generally state that
• DE hn

51
Types of Energy States
• Three types of energy states are usually
identified and used for the explanation of atomic
and molecular spectra
• Electronic Energy States??????? ?????? ????????
• These are present in all chemical species as a
consequence of rotation of electrons, in certain
orbits, around the positively charged nucleus of
each atom or ion.
• Atoms and ions exhibit this type of energy levels
only.

52
• 2. Vibrational Energy Levels
• ?? ???????? ???? ????? ??? ????? ???????These
are associated with molecular species only and
are a consequence of interatomic vibrations.
Vibrational energies are also quantized, that is,
only certain vibrations are allowed.
• 3. Rotational Energy Levels
• ???? ????? ?? ???????? ??????? ????? ????
???????
• These are associated with the rotations of
molecules around their center of gravities and
are quantized. Only molecules have vibrational
and rotational energy levels.

53
The solid black lines represent electronic energy
levels. Arrows pointing up represent electronic
absorption and arrows pointing down represent
electronic emission. Dotted arrows represent
relaxation from higher excited levels to lower
electronic levels. The figure to left represents
atomic energy levels while that to the right
represents molecular energy levels.
atoms
molecules
54
Line Versus Band Spectra
• Spectra Two types 1) Line Spectra 2) Band
Spectra
• 1) Line Spectra Resulted from atoms
• Atoms have electronic energy levels, absorption
or emission involves transitions between discrete
states with no other possibilities.
• 2) Band Spectra Resulted from molecules
• Molecular species contain vibrational and
rotational energy levels associated with
electronic levels, transitions can occur from and
to any of these levels.
• These unlimited numbers of transitions will give
an absorption or emission continuum, which is
called a band spectrum.

55
(No Transcript)
56
Black Body Radiation (Heated ??? ??????? ??????
??????? ????? ??????
• (Heated solids to incandescence)
• Gives continuum of radiation called black body
• Properties
• Dependent on the temperature ?max ?1/T
• As temperature of the emitting solid is
increased, the wavelength maximum is decreased.
• 2. The maximum wavelength emitted is independent
on the material from which the surface is made.
????? ???? ?????? ??? ??? ????? ??????? ???? ???
??????? ?????? ????? ??????

57
Increasing ??
58
The Uncertainty Principle????? ??? ??????
????????
• Werner Heisenberg, in 1927
• ?????? ???? ?????? ?????????? ?? ??? ??????
????? ????? ?? ?????
• ???? ???? ????????? ?????? ??? ??? ????
• (???? ??? ?? ????? ??????? ?????)
• Nature imposes limits on the precision with which
certain pairs of physical measurements can be
• This principle has some important implications
in the field of instrumental analysis and will be
referred to in several situations throughout the
course.

59
• Understand the meaning of this principle
• Comparing the two frequencies (Unknown and Known)
?1 and ?2 by measuring the difference (??).
• Now let both interfere to give a beat.
• The shortest time (?t) that can be allowed for
the interaction is the time of formation of one
single beat,
• ( equals to/or larger than the period of one beat
which is Pb)
• Therefore, we can write
• t ? Pb so, ? t ? 1/ ? ?

60
Multiply both sides by h
61
• Example
• The mean lifetime of the excited state when
irradiating mercury vapor with a pulse of 253.7
nm radiation is 210-8 s. Calculate the value of
the width of the emission line .
• ?Solution
• Line width can be measured from Uncertainty from
?.

62
?? c?-2? ?