Transcript Radiation and Spectra - Wayne State University

Radiation
&
Spectra
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Lite Question
What does it mean
to see something?
Astronomy and Light (1)
Most of the celestial objects studied in astronomy
are completely beyond human reach
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 veryfaint 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 the
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
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
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Detour: 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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Detour: 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 Charge
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,
not until the 19th century did scientist understand
that it was caused by moving charges
Thus, the electric charge is responsible for both
electricity and magnetism
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 conclusion:
Light is one form of a family of possible electric and
magnetic disturbances which travel and are called
electromagnetic radiation or electromagnetic waves
Experiments later confirmed Maxwell’s prediction
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Lite Question
What other waves do you know?
Electromagnetic Radiation
Electromagnetic (EM) radiation has some of the
characteristics that other types of waves have,
such as wavelength, frequency, and speed (see
next slide), as well as energy
Unlike most other kinds of waves, however, EM
waves can travel through empty space (vacuum)
Sound waves cannot travel through vacuum
Also unlike other types of waves, light and other
EM waves travel in empty space (vacuum) at the
same speed, which is the speed of light
The speed of light is 299,800 kilometers/second
This number is usually abbreviated as c
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Wave Characteristics
The wavelength () is
the size of one cycle
of the wave in space
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)
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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Electromagnetic Wave
The electric and magnetic fields of an EM wave 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
Animation
Visible Spectrum
Visible light (the EM radiation that the human eye
detects) has a range of wavelengths from 4,000
angstroms to 7,000 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
Simulations for combining light of different colors
When light rays pass from one transparent medium
(or a vacuum) to another, they are bent or refracted
The refraction angle depends on the wavelength (color)
In other words, light rays of different colors are bent
differently
Simulation
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
Simulation
First discovered by Newton
EM Radiation Carries Energy
Objects in the universe send us an
enormous range of EM radiation
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
Simulation
Visible Light
Since the speed of light is
v = c = 3 x 108 m/s, the
formula v = f x 
becomes c = f x 
c = f x  can be rewritten
as f = c/ or  = c/f
Thus, light with a larger wavelength has a lower
frequency, and light with a smaller wavelength has a
higher frequency
In the visible spectrum, red colors have the largest
wavelengths (lowest frequencies), whereas blue and
violet colors have the smallest wavelengths (highest
frequencies)
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 most radio waves reach the surface
More IR reaches elevations above 9,000 feet (2,765 meters)
The blocking of gamma rays, X-rays, and most UV by
the Earth’s atmosphere
is good for the preservation of life on the planet
but poses an obstacle to astronomers studying the sky
in these bands
Consequently, astronomers unable to detect these
types of radiation from celestial objects using groundbased instruments must perform their observations
from high mountaintops, high-flying airplanes, and
spacecraft
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Electromagnetic Spectrum & Earth’s Atmosphere
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Lite Question
Is light a wave or a particle?
Continuous Spectrum
This is a continuous band of the colors
of the rainbow, one color smoothly
blending into the next
A continuous spectrum is formed
whenever a solid, liquid, or very-dense
gas gives off radiation
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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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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)/ 
Thus, a high-energy EM wave has a high
frequency and a small wavelength
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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 and, therefore, “appears 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
Simulation
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
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
Line Spectra (1)
If a thin (low
density) gas is
heated until it
glows with its
own light, the
spectrum is not
continuous, but
consists of a
series of separate bright lines called emission lines
The lines imply that the atoms of the gas can emit only
certain discrete wavelengths (colors) of light
The gas of each particular element (such as hydrogen,
or sodium) produces an emission line spectrum that
has a specific pattern of lines unique to that element
and thus serves as its unique spectral signature
No two elements have the same patterns
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Line Spectra (2)
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
The combination
of dark lines and
continuous spectrum is called the absorption line
spectrum
The underlying continuous spectrum is produced by
the hotter and denser gas in the stars’ inner layers
The dark lines are produced by the cooler and
thinner gas in the stars’ outer layers, and imply
that the atoms of the gas can absorb only certain
discrete wavelengths (colors) of light
The gas of a particular element can produce both
emission and absorption line-spectra
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Absorption
& Emission
Line Spectra
Three Kinds of Spectra
Since each element has its own spectral signature in
the pattern of absorption or emission lines we
observe, spectral analyses can reveal some
information about the composition of the Sun and
other stars
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The Bohr Atom
Niels Bohr (1885-1962) developed
a model of the atom that provided
the explanation for 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 emit energy
The absorbed or emitted energy can be in the form
of a photon or an energy exchange with another
atom
Simulation
This model sounded outlandish, but numerous
experiments confirmed its validity
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 its own 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
&
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
Animations (for sound waves)
Doppler Effect in Radar Guns
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Doppler Shift in Spectra
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