Transcript The Spectrum

The Spectrum
What it is and what it means
This logo denotes A102 appropriate
Light and Sound
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To understand spectra you need a little
primer on light first
It is helpful to think of light and sound
together
They are both wave phenomena
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Sound is a mechanical wave
Light is a electromagnetic wave
You can think of color as pitch and brightness
as loudness
If the Rainbow was a Piano…
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Red would be on the low side of the scale (bass) and
violet would be on the high side (treble)
The range of colors is called the visible spectrum
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What would be lower than low?
What would be higher than high?
A Low Note
A High Note
Here’s what all possible pitches
together would sound like:
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What would it look like?
That’s Right!
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All possible colors added together with
the same brightness make white
I know, you’re thinking “but if I mix all
colors of paint together, it’s black.”
True, but paint doesn’t make light, it
absorbs light. If paint absorbs all colors,
what would you have?
Isaac Newton
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Showed with a prism*
that white light is a
continuous band made
of all colors
*How does a prism do this? I’ll explain the Physics after class if you wish.
Robert Boyle
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A contemporary and
adversary of Newton
Declared the prism to
be the "usefullest
Instrument" for gaining
insight into the fleeting
array of colors
generated when
sunlight passes through
it.”
William Herschel
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100 years after Boyle,
Herschel viewed starlight
through a prism
Saw differences in the
width and intensity of the
colors of the spectrum
Also detected “invisible
rays”
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Used a thermometer on
each color and found
“temperature” below red
Prisms
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Featured on the best
selling album of all time
Long before that,
though, scientists were
interested in improving
on the design
One goal was to
increase angular
dispersion
Why?
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The wider the angular dispersion, the
more spread out the colors are, and the
more detail is apparent
Prism improvers used various kinds of
glass, even liquids, and different
geometries to improve the resolution
Joseph von Fraunhofer
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German Optician b. 1787
At age 11, he was apprenticed to a glassmaker, Philipp Anton
Weichelsberger
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Weichelsberger’s shop collapsed, but both were rescued
Maximillian IV, Prince of Bavaria witnessed the rescue and took him under
his wing, providing him with books on physics and mathematics
Maximillian presented him with a sizable contribution to buy
his way out of apprenticeship at age 19, so he went to work
for lawyer named Joseph von Utzscneider who had
entrepreneurial aspirations
In partnership with Utzscneider he strived to make superior
optical devices, including better lenses for telescopes and
wide dispersion prisms
In 1814, searching for a pure light with which to calibrate his
instruments, Fraunhofer turned his prism to the Sun
What are these lines?
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Fraunhofer expected a pure, continuous
spectrum but was *surprised by the dark lines
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1821: Developed a superior diffraction grating
Invented the spectroscope
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Saw similar spectra in the light of Sirius
The basic design is still used
1822: Became keeper of the museum for the
Royal Academy of Sciences in Munich
Died 1826 of tuberculosis, age 39, before Bunsen
and Kirchhoff offered an explanation for the lines
later in the century
*William Hyde Wollaston saw the same lines in 1802 but
wasn’t impressed
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Even though Fraunhofer used his
spectroscope on a star*, there was no such
thing as stellar spectroscopy at the time
Astronomy was all about position, motion,
cataloging, discovery of new objects
Little or no significance was given to the
composition of celestial objects
*The Sun was not considered a star until later in the 19th century
Friedrich Wilhelm Bessel
(1784-1846)
[Astronomy] must lay
down the rules for
determining the motions of
the heavenly bodies as
they appear to us from the
earth…Everything else that
can be learned about the
heavenly bodies … is not
properly of astronomical
interest.
Other “expert” voices
In astronomy, human ingenuity will,
probably, in future, be able to
accomplish little more than an
improvement in the means of
making observations, or in the
analysis by which the rules of
computation are investigated.
—John Narrien (1833)
We can imagine the possibility of determining the
shapes of stars, their distances, their sizes, and their
movements;...whereas there is no means by which
we will ever be able to examine their chemical
composition, their mineralogical structure, or
especially, the nature of organisms that live on their
surfaces…Our positive knowledge with respect to the
stars is necessarily limited to their observed
geometrical and mechanical behavior.
—Auguste Comte (1864)
Remember this!
However, the game was afoot
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Alexander von Humbolt
studies sunspots
42 years of
observations revealed
that sunspot activity
varies on an 11-year
cycle
Right: 1845
daguerreotype, perhaps
the first photograph of
the Sun
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Note the sunspots
Earth-Sun connection
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Sir Edward Sabine in
1852 announces a
remarkable coincidence
between Earth’s
magnetic fluctuations
and the sunspot cycle
If the Sun could
influence the Earth’s
magnetic field, and
hence navigational
compasses, more
intense solar studies
were indicated
Physics and Chemistry Unite
Gustav Robert Kirchhoff 1824-1887
Robert Wilhelm Bunsen 1811-1899
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In the 1850s Kirchhoff and Bunsen used a
prism to examine the light produced when
different elements, alone and in combination,
are burned
“It is known that several substances have the
property of producing certain bright lines
when brought into the flame. A method of
qualitative analysis can be based on these
lines, whereby the field of chemical reactions
is greatly widened and hitherto inaccessible
problems are solved.”
prism
collimating tube
eyepiece
sample
Burner
(yes, a Bunsen burner!)
Spectral Lines…
Fraunhofer’s Lines…
Spectral Lines…
Fraunhofer’s Lines: Hmmmm!
An answer to an old question
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From this work B&K understood what the
Fraunhofer lines were
‘the vapour of table salt absorbs the same
lines which it also emits. These lines are
identical with solar Fraunhofer lines’.
So, there is a connection between emission
and absorption!
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And stars??
Warning! Science Content Follows!
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Every element (and molecules too)
emits a unique set of spectral lines
It’s like every chord on the piano has a
unique set of pitches
But two questions come to mind:
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Why does each element emit a unique set
of spectral lines?
Why did it take so long for scientists to
concertedly look for these lines?
Second Answer First
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At the time of Newton’s death in 1727, only
14 of the 114 elements had been identified
Even by 1800, only 32 elements had been
isolated
But by 1859, the time of Bunsen and
Kirchhoff’s paper, known elements were so
numerous that chemists were eager to qualify
and quantify them
And, of course, Astronomers weren’t
interested at all
Now, the First Answer
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This interesting phenomenon,
seemingly pertinent only to chemical
identification, was one of the motivating
forces for a major change in Physics in
the 19th and 20th Centuries
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“Clouds over Physics”
“Electromagnetic Catastrophe”
Faraday and Maxwell
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It was known
through the work of
Michael Faraday and
James Clark Maxwell
that moving
electrons radiate
energy
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This is a slightly
more modern picture
of an atom
But the (19th C) Problem is:
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If an electron is continually buzzing around
an atom, it is constantly giving off energy
If it is constantly giving off energy, its path
would decay and the electron would spiral
into the nucleus
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The electron would emit the entire spectrum as it
spins down, not discrete colors
And atoms and therefore matter everywhere
would collapse
Max Planck’s Kludge
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Planck suggested that
electrons can travel only
in specific paths or orbits
about the nucleus
That way they would
never lose all their energy
and spiral in
He admitted that the
atom probably wasn’t like
this, but the math worked
out as if it did
But Atoms Do Work This Way!
Orbits and Spectra
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Every element’s atoms have a unique set of discrete
energy levels (orbits)
As the electron drops from a higher to lower orbit it
sheds a unique color of light
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Small energy jumps: red light
Big energy jumps: violet light
The set of colors (a series) gives each atom its
distinctiveness
AND, if a photon of just the right energy hits a cool,
rarified gas, the gas absorbs that color
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Fraunhofer lines!
Without going into great detail…
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Planck contributed to a new science called
Quantum Mechanics (term dates from 1927)
Quantum Mechanics was a sea change in the
way scientists (and non-scientists) saw the
cosmos
Newton’s idea of a deterministic universe was
supplanted by a probabilistic universe
In other words, there is no absolute certainly
(I think)
A Powerful Technique
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Bunsen and Kirchhoff said something to the
effect that, if Heidelberg was burning they
could determine what is was made of
They then metaphorically looked at each
other and realized they could tell what the
Sun was made of*!
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And they found it wasn’t burning in the “normal”
sense
As an aside, contemporaries Herman von
Helmholtz and Lord Kelvin postulated that the Sun
shined from a release of gravitational energy
*Of course, others had this idea as well, but B & K are best know for it.
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If we were to go to the sun, and to bring
away some portions of it and analyze them in
our laboratories, we could not examine them
more accurately than we can by this new
mode of spectrum analysis.
—Warren De La Rue (1861)
Kirchhoff’s Laws
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After further study Kirchhoff postulated
three laws:
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A rarified hot gas gives off emission
spectra
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A dense cool gas absorbs light at discrete
colors (absorption spectra)
A dense hot gas emits a continuous
spectrum
A New Science: Astrospectroscopy
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Astronomers break the
light from stars,
nebulae, and
supernovae into its
constituent colors
Using Kirchhoff’s Laws
they can tell what a
hugely distant object is
made of, whether it is
hot, cold, rarified or
dense
Pioneers in Stellar Spectroscopy
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Giovanni Battista Donati (1826-1873)
Father Pietro Angelo Secchi (18181878)
Lewis Morris Rutherfurd (1816-1892)
Comparing stellar spectra
Donati (1863)
N.B. These and those that follow
are hand-drawn spectragrams
Comparing stellar spectra
Rutherfurd (1863)
Comparing stellar spectra
Greenwich Observatory (1863)
William Huggins
1824-1910
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English amateur Astronomer
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Semi-pro might be a better term
Amateur at that time did not mean someone
without considerable skills
Non-professionals discovered several moons
around Saturn and Uranus
Huggins didn’t start observing until he was 30
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He closed the family shop in London and moved
back in with his parents in Upper Tulse Hill
Title page of Huggin’s lab
notebook #1
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For several years he
worked in his
observatory on
Tulse Hill outside
London
But Huggins grew
dissatisfied with the
positional focus of
19th century
Astronomy
Epiphany
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“It was just about this time … that the news
reached me of Kirchhoff's great discovery of
the true nature and the chemical constitution
of the sun from his interpretation of the
Fraunhofer lines….”
He later wrote: ...This news was to me like
the coming upon a spring of water in a dry
and thirsty land. Here at last presented itself
the very order of work for which in an
indefinite way I was looking….
William Allen Miller (1817-1870)
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Trained as a chemist
Founding member of the
Chemical Society
Treasurer and VP of the
Royal Society
Started collaborating with
Huggins in 1862
Both were members of the
Royal Astronomical Society
The two jointly won the Gold
Medal in 1867 for their work
on stellar spectra
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Worked to improve prismatic analysis with the
goal of identification of elements
Induction apparatus to examine metallic spectra
Huggins’
star-spectroscope
Huggins’ High Dispersion Spectrascope
Comparing stellar spectra
Huggins and Miller, Proc. Roy. Soc. (1863)
Sun
Betelgeuse
Sirius
Aldebaran
The stars were undoubtedly suns after the
order of our sun*…
—Huggins (1897)
*What was it that Giordano Bruno had conjectured?
Interpreting nebular spectra
Huggins and Huggins, Proc. Roy. Soc. (1889)
Remember: these are hand-drawn spectragrams
Other Stellar Mysteries
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Huggins observed the
planetary nebula 37. H
IV Draconis
… after a few moments
of hesitation, I put my
eye to the
spectroscope. Was I
not about to look into a
secret place of
creation?... I looked
into the spectroscope.
No spectrum such as I
expected! A single
bright line only!
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The bright line was something new in the spectrum
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Huggins called it nebulium
In a planetary nebula, ionized oxygen and other elements
are blasted off a giant star
This is the line ionized oxygen makes, not reproducible in
19th century chemistry labs
Margaret Lindsay Huggins (1848-1915)
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Married William when
he was around 50
Margaret was an
accomplished
photographer
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She designed
photographic equipment
for use with the
spectroscope
Huggins laboratory
notebooks also become
much more organized
and informative due to
Margaret
William Huggins
in the
Tulse Hill Observatory
(ca. 1905)
The Modern Variety of Data
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The data plots don’t necessarily look like rainbows
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Infrared, gamma-ray, microwave, x-ray spectra are all
valuable data
The discovery of helium
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P.J.C. Janssen (Fr) and J. Norman Lockyer (UK) independently
determined that the solar atmosphere could be studied
spectroscopically
Consequently, the bright orange ‘D’ line that was assumed to be
sodium turned out to be an indication of a new element
Helium isolated in the laboratory in 1895 by William Ramsay
VVVVVrrrrroooooommmm
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Spectra can also tell Astronomers if a
distant object is moving towards or
away from us
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And also how fast it is spinning
The technique is to use the Doppler
Effect to see how the spectral lines are
affected
To sum up that animation (again)
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Because light waves travel only at a
fixed speed, a light-emitting object like
a star will be “redder” if it moves away
from us and “bluer” if it moves towards
us, meaning that the pattern of spectral
lines will shift to the red or blue but
maintaining their relative positions to
each other
And you find it in classic rock!
Armand Hippolyte Louis Fizeau (September 23,
1819 – September 18, 1896
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First predicted red shift
shortly after Doppler
discovered it
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Also showed that two
telescopes could be
combined, forming a
single, much larger
aperture
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Interferometry
Red Shift
Huggins had noted this in 1868
Doppler widening
Spin
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Astronomers can tell if a distant galaxy is spinning
and how fast
The light on one edge is blue (moving towards us)
and the other edge is red (moving away)
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I p-shopped this up to make a point 
What was that about a bad neighborhood?
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In the 1920s Edwin
Hubble examined the
spectra of stars of many
galaxies
He did his work at the
Wilson Observatory
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Here he is with his pipe
You can almost hear his
British accent
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(He was born in
Marshfield, Missouri)
An Expanding Universe
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Hubble discovered
that the more
distant a galaxy was
from us, the faster it
was moving away
Can only be
explained by an
expanding Universe
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A lecture for another
day