The Spectrum What it is and what it means This logo denotes A102 appropriate.
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Transcript of The Spectrum What it is and what it means This logo denotes A102 appropriate.
The Spectrum
What it is and what it means
This logo denotes A102 appropriate
Light and Sound 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
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…
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 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:
What would it look like?
That’s Right!
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 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 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
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”
Used a thermometer on each color and found “temperature” below red
Prisms
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?
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 German Optician b. 1787 At age 11, he was apprenticed to a glassmaker,
Philipp Anton Weichelsberger 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?
Fraunhofer expected a pure, continuous spectrum but was *surprised by the dark lines
Saw similar spectra in the light of Sirius 1821: Developed a superior diffraction grating Invented the spectroscope
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
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
[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.
Friedrich Wilhelm Bessel(1784-1846)
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)
Other “expert” voices
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 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
Note the sunspots
Earth-Sun connection 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
Robert Wilhelm Bunsen 1811-1899
Gustav Robert Kirchhoff 1824-1887
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.”
sample
prism
collimating tube
eyepiece
Burner (yes, a Bunsen burner!)
Spectral Lines…
Fraunhofer’s Lines…
Spectral Lines…
Fraunhofer’s Lines: Hmmmm!
An answer to an old question 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! And stars??
Warning! Science Content Follows!
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: 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 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
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 “Clouds over Physics” “Electromagnetic Catastrophe”
Faraday and Maxwell
It was known through the work of Michael Faraday and James Clark Maxwell that moving electrons radiate energy This is a slightly
more modern picture of an atom
But the (19th C) Problem is:
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 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
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
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
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
Fraunhofer lines!
Without going into great detail…
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 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*! 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.
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
After further study Kirchhoff postulated three laws: A rarified hot gas gives off emission
spectra A dense cool gas absorbs light at
discrete colors (absorption spectra) A dense hot gas emits a continuous
spectrum
A New Science: Astrospectroscopy
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
Giovanni Battista Donati (1826-1873) Father Pietro Angelo Secchi (1818-
1878) Lewis Morris Rutherfurd (1816-1892)
Comparing stellar spectraDonati (1863)
N.B. These and those that follow are hand-drawn spectragrams
Comparing stellar spectraRutherfurd (1863)
Comparing stellar spectraGreenwich Observatory (1863)
William Huggins1824-1910
English amateur Astronomer 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 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
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 “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) 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
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 spectraHuggins 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 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!
The bright line was something new in the spectrum 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)
Married William when he was around 50
Margaret was an accomplished photographer
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
The data plots don’t necessarily look like rainbows
Infrared, gamma-ray, microwave, x-ray spectra are all valuable data
The discovery of helium 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
Spectra can also tell Astronomers if a distant object is moving towards or away from us 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)
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
First predicted red shift shortly after Doppler discovered it
Also showed that two telescopes could be combined, forming a single, much larger aperture
Interferometry
Red Shift
Huggins had noted this in 1868
Doppler widening
Spin 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) I p-shopped this up to make a point
What was that about a bad neighborhood?
In the 1920s Edwin Hubble examined the spectra of stars of many galaxies
He did his work at the Wilson Observatory
Here he is with his pipe
You can almost hear his British accent
(He was born in Marshfield, Missouri)
An Expanding Universe 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 A lecture for
another day