Transcript Radiation and Spectra - Wayne State University

Radiation and Spectra
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Lite Question
What does it mean to see something?
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Astronomy and Light (1)
Most of the celestial objects studied in
astronomy are completely beyond human reach
The astronomers gain information about them
almost exclusively through the light and other
kinds of radiation received from them
Light is the most familiar form of radiation, which
is a general term for (electromagnetic) waves
Because of this fact, astronomers have devised
many techniques to decode as much as possible
the information that is encoded in the often very
faint rays of light from celestial objects
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Astronomy and Light (2)
If this “cosmic code” can be deciphered, we can
learn an enormous amount about astronomical
objects (their composition, motion, temperature,
and much more) without having to leave Earth
or its immediate environment!
To uncover such information, astronomers must
be able to analyze the light they receive
One of astronomers’ most powerful tools in
analyzing light is spectroscopy
This is a technique of dispersing (spreading out) the
light into its different constituent colors (or
wavelengths) and analyzing the spectrum, which is
the array of colors
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Astronomy and Light (3)
Physicists have found that light and other
types radiation are generated by processes
at the atomic level
Thus, to appreciate how light is generated
and behaves, we must first become
familiar with how atoms work
Our exploration will focus on one particular
component of an atom, called electric
charge
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Electric Charge
Many objects have not only mass, but also an
additional property called electric charge, which can
be traced to the atoms that the objects are made of
In the vicinity of an electric charge, another charge
feels a force of attraction or repulsion
This is true regardless of whether the charges are at
rest or in motion relative to each other
There are two kinds of charge: positive and negative
Like charges repel, and unlike charges attract
If the charges are in motion relative to each other,
another force arises, which is called magnetism
Although magnetism was well known for millennia, its
being caused by moving charges was not understood
until the 19th century
Thus, the electric charge is responsible for both
electricity and magnetism
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The Atom and the Nucleus
Each atom consists of a core, or nucleus, containing
positively charged protons and neutral neutrons, and
negatively charged electrons surrounding the nucleus
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Isotopes of Hydrogen
The hydrogen atom is the simplest,
consisting of only one proton and
one electron
Although most hydrogen atoms
have no neutrons at all, some may
contain a proton and one or two
neutrons in the nucleus
The different hydrogen nuclei with different numbers
of neutrons are
called isotopes of
hydrogen
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Electric and Magnetic Fields
In physics, the word field (or force field)is
used to describe the action of forces that one
object exerts on other distant objects
For example, the Earth produces a gravitational
field in the space around it that controls the
Moon’s orbit about Earth, although they do not
come directly into contact
Thus, a stationary electric charge produces
an electric field around it, whereas a moving
electric charge produces both an electric field
and a magnetic field
Similarly, a magnet is surrounded by a
magnetic field
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James Clerk Maxwell (1)
Maxwell (1831-1879), born and
educated in Scotland, unified the
rules governing electricity and
magnetism into a coherent theory
It describes the intimate relationship between
electricity and magnetism with only a few elegant
formulas
Also, it allows us to understand the nature and
behavior of light
Before Maxwell proposed his theory, many
experiments had shown that changing magnetic
fields could generate electric fields
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James Clerk Maxwell (2)
Maxwell’s theory led to a hypothesis:
If a changing magnetic field can create an electric
field, then a changing electric field can create a
magnetic field
The consequences of his hypothesis:
Changing electric and magnetic fields should trigger
each other
The changing fields should spread out like a wave and
travel through space at a speed equal to the speed of
light
Maxwell’s concluded:
Light is one form of a family of possible electric and
magnetic disturbances which travel called
electromagnetic radiation or electromagnetic waves
Experiments later confirmed Maxwell’s prediction
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Electromagnetic Radiation (1)
Electromagnetic (EM) radiation has some of the
characteristics that other types of waves have,
such as wavelength, frequency, and speed (see
next slide)
Unlike most other kinds of waves, however, EM
waves can travel through empty space (vacuum)
Sound waves cannot travel through vacuum
The speed of light, and other EM radiation, is
constant in empty space
All forms of radiation have the same speed of
299,800 kilometers/second in vacuum
This number is abbreviated as c
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8
Wave Characteristics
It is also the distance from
one crest (or one trough) to
the next
Common units for  are
meter (m), nanometer (nm),
and angstrom (A)
4
Wave Amplitude
The wavelength () is the
size of one cycle of the wave
in space
Wavelength
6
2
0
0
5
10
15
20
-2
-4
-6
-8
Distance (m)
The frequency (f) of the wave
indicates the number of wave cycles that pass per second
The unit for frequency is hertz (Hz)
The speed (v) of the wave indicates how fast it
propagates through space
Common units for v are m/s, km/hour, and miles/hour
v=fx
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The electric and magnetic fields oscillate at right angles
to each other and the combined wave moves in a
direction perpendicular to both of the electric and
magnetic field oscillations.
Electromagnetic Radiation (2)
Visible light (what your eye detects) has a
range of wavelengths from 4000 angstroms
to 7000 angstroms (or from 400 nm to
700 nm)
1 angstrom = 10-10 meter
Different wavelengths of light are perceived
by the eye as different colors
White light is a combination of all the colors
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Refraction of Light
When light rays pass from one transparent medium
(or a vacuum) to another, the rays are bent or
refracted
The refraction angle depends the wavelength (color)
In other words, light rays of different colors are bent
differently
Incidence angle
Refraction angle
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Dispersion by Refraction
The separation of light into its various colors is
called dispersion
White light passing through a prism undergoes
dispersion into different colors
What is produced is a rainbow-colored band of light
called a continuous spectrum
First discovered by Newton
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EM Radiation Carries Energy
The types of radiation, from the
highest to lowest energy, are
Gamma rays
X-rays
Ultraviolet (UV)
Visible light
Infrared (IR)
Radio waves
Microwaves are high-energy radio waves
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Electromagnetic Spectrum
The EM spectrum is the entire range of wavelengths
of EM radiation, including the visible spectrum
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Period/Frequency Examples
Phenomenon
Earth's Orbit
around Sun
Earth Rotation
Electrical Power
(US)
Light(Blue)
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Period
365
31536000
1
86400
Frequency
days
s
days
s
0.01666667 s
1.6667E-15 s
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0.00273973
3.171E-08
1
1.1574E-05
/day
Hz
/day
Hz
60 Hz
6E+14 Hz
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Visible Light (1)
Since the speed of light
is v = c = 3 x 108 m/s,
the formula v = f x 
becomes
c=fx
c = f x  can be rewritten as
f = c/
 = c/f
Light with a smaller wavelength has a
higher (larger) frequency
Light with a longer wavelength has a lower
(smaller) frequency
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Visible Light (2)
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color

violet
f (*1014 Hz)
Energy (*10-19 J)
4000 - 4600
7.5 - 6.5
5.0 - 4.3
indigo
4600 - 4750
6.5 - 6.3
4.3 - 4.2
blue
4750 - 4900
6.3 - 6.1
4.2 - 4.1
green
4900 - 5650
6.1 - 5.3
4.1 - 3.5
yellow
5650 - 5750
5.3 - 5.2
3.5 - 3.45
orange
5750 - 6000
5.2 - 5.0
3.45 - 3.3
red
6000 - 8000
5.0 - 3.7
3.3 - 2.5
(angstroms)
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Electromagnetic Radiation
Reaching Earth
Not all wavelengths of light from space
make it to Earth’s surface
Only long-wave ultraviolet (UV), visible,
parts of the infrared (IR), and radio waves
make it to surface
More IR reaches elevations above
9,000 feet (2,765 meters) elevation
This is one reason why modern
observatories are built on top of very high
mountains
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Earth’s Atmosphere
Blocks gamma rays, X-rays, and most UV
Good for the preservation of life on the planet…
An obstacle for astronomers who study the sky in
these bands
Blocks most of the IR and parts of the radio
Astronomers unable to detect these forms of
energy from celestial objects from the ground
Must resort to very expensive satellite
observatories in orbit
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Electromagnetic Spectrum and
Earth’s Atmosphere
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Lite Question
Is light a wave or a particle?
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Max Planck’s Photon
Planck (1858-1947) discovered
that if one considers light as
packets of energy called
photons, one can accurately
explain the shape of continuous spectra
A photon is the particle of electromagnetic
radiation
Bizarre though it may be, light is both a
particle and a wave
Whether light behaves like a wave or like a
particle depends on how the light is
observed
This depends on the experimental setup!
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A Continuous Spectrum
This is a continuous band of the colors
of the rainbow, one color smoothly
blending into the next
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Albert Einstein’s Photon
Energy Interpretation
A few years after Planck's
discovery, Einstein (1879-1955)
found a very simple relationship between
the energy of a light wave (photon) and its
frequency (f)
Energy of light = h × f
Here h = 6.63 × 10-34 J·sec is a universal
constant of nature called Planck's constant
Alternatively, energy of light = (h × c)/ 
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Blackbody Radiation
A blackbody is an idealized object which absorbs all
the electromagnetic radiation that falls on it,
reflecting none of the incoming radiation
In other words, a blackbody is a perfect absorber of
radiation, thus “appearing black”
When a blackbody is heated, it emits EM radiation
very efficiently at all wavelengths
A blackbody is thus an excellent emitter of radiation
Though no real object is a perfect blackbody, most
celestial bodies behave very much like a blackbody
when it comes to emitting radiation
In other words, they produce radiation spectra that are very
similar to the spectrum of blackbody radiation
Therefore, understanding the blackbody spectrum
allows us to understand the radiation from celestial
objects
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Blackbody
Spectrum (1)
These graphs
show that the
higher the
temperature of
a blackbody,
the shorter the
wavelength at
which maximum power is emitted
Power is the amount of energy released per second
The wavelength (max) at which maximum power is
emitted by a blackbody is related to its kelvin
temperature (T) by max = 3 x 106/T
This relationship is known as Wien’s law
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Blackbody
Spectrum (2)
These graphs
also show that a
blackbody (BB)
at a higher
temperature
emits more
power at all
wavelengths than does a cooler BB
The total power emitted per unit area (F) by a BB is
proportional to its kelvin temperature (T) raised to
the fourth power, namely F  T4
This is known as the Stefan-Boltzmann law
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Star Color and
Temperature
Lessons learned
from blackbody
radiation can be
used to estimate
the temperature of
stars and other
celestial bodies
Thus, the dominant
color and the
brightness of a
body can give us
some idea about its
temperature
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Discrete Spectra
A close examination of the spectra from the
Sun and other stars reveals that the rainbow
of colors in their spectra has many dark
lines, called absorption lines
They are produced by the cooler thin gas in the
upper layers of the stars absorbing certain colors
of light produced by the hotter dense lower layers
The spectra of hot, thin (low density) gas
clouds are a series of bright lines called
emission lines
In both of these types of spectra you see
spectral features at certain, discrete
wavelengths (or colors) and nowhere else
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Absorption
and
Emission
Line
Spectra
Spectra (1)
The type of line spectrum you see depends on the
temperature of the thin gas
If the thin gas is cooler than the thermal source in the
background, you see absorption lines
Since the spectra of stars show absorption lines, it tells you
that the density and temperature of the upper layers of a
star is lower than the deeper layers
In a few cases you can see emission lines on top of a
continuous spectrum — this is produced by a thin gas that is
hotter than the thermal source in the background
The spectrum of a hydrogen-emission nebula (= gas or
dust cloud) is just a series of emission lines without any
continuous spectrum because there are no stars visible
behind the hot nebula
Some objects produce spectra that are a combination of
a continuous spectrum, an emission-line spectrum, and
an absorption-line spectrum simultaneously!
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Spectra (2)
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The Bohr Atom
Niels Bohr (1885-1962) developed
a model of the atom that provided
the explanation for discrete-line
spectra in the early 20th century
In the model, an electron can be
found only in energy orbits of certain sizes
Also, if the electron moves from one orbit to
another, it must absorb or radiate energy
The absorbed or radiated energy can be in the
form of a photon or an energy exchange with
another atom
This model sounded outlandish, but numerous
experiments confirmed its validity
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Bohr’s Model of the Atom
The massive but small positively-charged protons and
massive but small neutral neutrons are found in the tiny
nucleus
The small negatively-charged electrons move around the
nucleus in certain specific orbits (energies)
An electron is much lighter than a proton or neutron
In a neutral atom the number of electrons equals the number of
protons
The arrangement of an atom's energy orbits depends on the
number of protons and neutrons in the nucleus and the
number of electrons orbiting the nucleus
Each type of atom has a unique arrangement of the energy
orbits and, therefore, produces its own unique pattern of
emission or absorption lines
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How Emission Line is Produced
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Spectral
“Signatures”
of
Hydrogen
and
Helium
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How Absorption Line is Produced
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Doppler Effect When Source and
Observer are in Relative Motion
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No Doppler Effect When Source and
Observer are not in Relative Motion
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Doppler Effect in Radar Guns
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Doppler Shift in Spectra
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Doppler Shift in Radiation Graphs (1)
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Doppler Shift in Radiation Graphs (2)
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