Kirchhoff's Laws of Spectroscopy
Summary
TLDRIn this introductory astronomy lecture, Jason Kendall explores spectroscopy, the study of light's interaction with matter. He explains how a prism can create a spectrum, revealing absorption and emission lines that act as unique fingerprints for elements. Kendall discusses Gustaf Kirchhoff's laws, which connect emission and absorption spectra, and how they helped transform astronomy into astrophysics. The lecture also touches on how spectroscopy reveals the composition, temperature, and physical conditions of celestial objects, allowing us to understand the universe's distant phenomena.
Takeaways
- ๐ Spectroscopy is the study of how light interacts with matter and how this interaction is transmitted through space.
- ๐ฌ By breaking down light into a spectrum, we can analyze the intensity of light varying with wavelength or frequency.
- ๐ A prism can be used to create a spectrum from white light, showing a rainbow effect which is easier to see when light passes through a narrow slit.
- ๐ The spectrum obtained from a prism shows the interaction of light with matter, and in the case of sunlight, it displays absorption lines which are dark spots against the rainbow.
- ๐ The Sun's spectrum is an absorption spectrum, showing light absorbed from the continuous background.
- ๐ก Emission spectra occur when a material emits light at specific frequencies, creating bright lines on a dark background.
- ๐ Elements emit specific emission spectra when heated, regardless of their source, providing a unique fingerprint for identification.
- ๐ฌ Kirchhoff's laws of spectroscopy describe the relationship between emission and absorption spectra and the conditions under which they occur.
- ๐ก Spectroscopy can reveal the temperature of a gas by how it absorbs or emits light at different wavelengths or frequencies.
- ๐ The broadening of spectral lines can indicate pressure and density conditions in the emitting gas, such as pressure broadening due to high speeds of atoms.
- ๐ Spectroscopy transformed astronomy into astrophysics by allowing scientists to understand the composition and physical conditions of stars and other celestial objects.
Q & A
What is spectroscopy?
-Spectroscopy is the study of how light interacts with matter and how this interaction is transmitted through space to us, allowing us to analyze the intensity of light as it varies with wavelength or frequency.
How does a prism create a spectrum?
-A prism creates a spectrum by breaking apart light due to the different speeds of light for different wavelengths within the glass, which spreads the light into its constituent wavelengths or frequencies.
What is the difference between absorption lines and emission lines?
-Absorption lines are dark spots in a spectrum where light has been absorbed, making it dimmer than the surrounding continuum. Emission lines, on the other hand, are bright lines on a dark background where specific wavelengths of light are emitted by a material.
What is the significance of the solar spectrum having absorption lines?
-The solar spectrum having absorption lines indicates that certain wavelengths of light are being absorbed by elements in the Sun's atmosphere, which can reveal the composition of the Sun.
Why do different elements produce different emission spectra?
-Different elements produce different emission spectra because each element has unique atomic structures that emit light at specific wavelengths or frequencies when heated.
What are Kirchhoff's laws of spectroscopy?
-Kirchhoff's laws of spectroscopy state that: 1) Emission lines are produced at single frequencies against a dark background from a hot, rarified gas. 2) A hot, opaque body emits a continuous spectrum. 3) A hot, opaque body with cool gas in front of it will show an absorption spectrum when viewed through the gas.
How can spectroscopy be used to determine the composition of a distant object?
-Spectroscopy can determine the composition of a distant object by analyzing the emission or absorption lines in the object's spectrum, which act as fingerprints for different elements.
What is the significance of emission spectra being the same regardless of the source of the material?
-The fact that emission spectra are the same regardless of the source indicates that the nature of matter and its interaction with light is consistent across the universe, providing a reliable method for identifying elements.
How does the temperature of a gas affect its spectrum?
-The temperature of a gas affects its spectrum by influencing the energy levels of its atoms, which in turn affects the wavelengths of light that are absorbed or emitted.
What is pressure broadening in spectroscopy?
-Pressure broadening is a phenomenon where emission lines become broader under high pressure due to the increased speed of atoms, causing a Doppler shift that affects the width of the spectral lines.
How does spectroscopy contribute to our understanding of astrophysics?
-Spectroscopy contributes to astrophysics by allowing us to analyze the composition, temperature, and other physical properties of celestial objects from the light they emit or absorb, despite being far away.
Outlines
๐ Introduction to Spectroscopy
Jason Kendall introduces the concept of spectroscopy, which is the study of how light interacts with matter. He explains that spectroscopy allows us to analyze light from distant sources by breaking it into a spectrum to observe how its intensity varies with wavelength or frequency. Kendall uses the analogy of a prism creating a rainbow effect from white light to illustrate dispersion of light. He further discusses how passing light through a narrow slit and then a prism can help in analyzing the light's interaction with matter, leading to the observation of absorption and emission lines in a spectrum. The paragraph concludes with examples of absorption lines in the sun's spectrum and the concept of emission spectra from materials like sodium and neon.
๐ฌ The Significance of Emission Spectra
The paragraph delves into the uniqueness of emission spectra, highlighting that every element, regardless of its origin, emits light at specific wavelengths when heated, creating a fingerprint or signature. This property is crucial for identifying elements. The discussion then moves to Kirchhoff's laws of spectroscopy, which were empirically derived in the 19th century and later explained by quantum mechanics. These laws describe the relationship between emission and absorption spectra and the conditions under which they occur. The paragraph emphasizes how spectroscopy provides insights into the nature of matter and the importance of understanding these laws for the development of astrophysics.
๐ Advanced Spectroscopy Techniques
This section discusses the practical applications of spectroscopy, including the analysis of ionized gases and molecular spectra. It explains how the spectrum changes with ionization and the complexity introduced when dealing with molecules. The paragraph also touches on the discovery of simple molecules like ethyl alcohol and amino acids in space, illustrating the capability of spectroscopy to reveal the composition of distant objects without direct contact. Furthermore, it discusses how the study of line broadening due to pressure and density can provide information about the conditions in a star's atmosphere.
๐ The Impact of Spectroscopy on Astrophysics
The final paragraph emphasizes the transformative role of spectroscopy in astronomy, leading to the field of astrophysics. It explains how the analysis of starlight through spectroscopy allows scientists to understand the physical conditions and compositions of stars and other celestial bodies. The paragraph also discusses the Doppler shift and how it affects the observed spectra, providing information about the motion of stars and gas clouds. The universality of physical laws is highlighted as a foundational assumption that enables the interpretation of spectroscopic data from distant objects across the universe.
Mindmap
Keywords
๐กSpectroscopy
๐กPrism
๐กSpectrum
๐กAbsorption Lines
๐กEmission Lines
๐กContinuum
๐กKirchhoff's Laws
๐กFingerprint
๐กIonized Gas
๐กDoppler Shift
๐กAstrophysical
Highlights
Spectroscopy is the study of how light interacts with matter and how this interaction is transmitted through space.
Spectroscopy involves breaking light from a distant source into a spectrum to analyze its intensity variation with wavelength or frequency.
A prism can be used to create a spectrum by passing light through it, separating it into constituent wavelengths.
A narrow slit is used to create a beam of light for more precise spectroscopic analysis.
The rainbow effect from sunlight passing through a prism is called the Continuum, with darker spots known as absorption lines.
The Sun's spectrum is an absorption spectrum, indicating light absorbed from the Continuum.
Emission spectra occur when a material emits light at specific frequencies, creating bright lines on a dark background.
Each chemical element has a unique emission spectrum, acting as a fingerprint.
Gustaf Kirchhoff's laws of spectroscopy relate to the emission and absorption of light by matter.
Kirchhoff's first law states that emission lines are produced at single frequencies against a dark background.
Kirchhoff's second law explains how a continuous spectrum is absorbed by cooler gas, creating an absorption spectrum.
Emission and absorption lines occur at the same wavelengths, indicating the nature of matter.
Spectroscopy can reveal the composition of gases by identifying their unique spectral fingerprints.
The spectral lines' profile can provide information about the physical conditions of the emitting or absorbing matter.
Pressure broadening and density broadening affect the width of spectral lines, providing insights into the matter's state.
Spectroscopy transformed astronomy into astrophysics by allowing the understanding of star compositions and conditions.
The universality of the laws of physics is fundamental to interpreting spectroscopic data from distant objects.
Transcripts
hello this is Jason Kendall and welcome
to the next of my introductory astronomy
lectures today we're going to be talking
about spectroscopy spectroscopy is the
study of of how light interacts with
matter and how light the interaction of
matter with light betrays itself and
gets transmitted through space to us so
we can receive it spectroscopy is simply
the way we take the light that's coming
from a distance source and use some
method by which we can break it apart
into a spectrum to see how the intensity
of the light varies with wavelength or
frequency so an easy way to think about
this is allow normal white light to pass
through a prism and then you'll see a
rainbow effect well typically a rainbow
effect is pretty hard to see unless of
course there's a very narrow opening
through which that that passes the light
in a very narrow beam and that narrow
beam then goes through the prism if it's
a general wave of light that goes
through it or an OM
then the prism will not necessarily be
create a spectrum well it will create a
spectrum it be harder to see so for us
to make it better what we do is we take
the source whatever it is send it
through a narrow slit so that it becomes
a ray or a beam we don't care about
losing all that light because we're just
going to analyze the light that comes to
us from there we pass it through a prism
the prism breaks apart the light by
because the inside of glass the speed of
light is different for different
wavelengths and so it spreads the light
into its constituent wavelengths or
frequencies and then we take a picture
of the incident light on out after it
goes through the prism and that is the
study of spectroscopy something divides
the light up so that we can see the
intensity of given wavelengths or
frequencies and that we know exactly how
that thing is interacting with the light
meaning the prism so we don't have to
worry about the prism's interaction of
the light we just worry about the origin
of the source all right so if we then
look at more common things like if we
take a prism just a garden household
variety prism or a crystal you notice
that there's always a pretty rainbow
that comes off of the sunlight well
pretty rainbows from the sunlight are
indicated here in this image from the
McMath solar Observatory down in Tucson
Arizona run by noo and this this absorb
this rainbow that we always see has
actually some darker spots in it and
those darker spots are the Shadows of
the slit through which we passed the
sunlight so that's why we call them
absorption lines because we make a we
make a line a a vertical line slit
through which we pass the solar the
sunlight and that sunlight then passes
into our spectroscope and that
spectroscope might just simply be a
prism or it might be some other device
like in a shell spectrograph or
something like that or reflecting off of
the surface but at the angle what we see
is we see a series of darks lines on the
Spectrum and those are places where it
simply is dimmer than the surrounding in
Continuum so the rainbow itself we call
it the Continuum or the continuous
spectrum and there are absorption
features which are darker than the
surrounding Continuum so the sun's
spectrum is actually what we call an
absorption Spectrum an absorb there is
light that's been absorbed from the
Continuum all right so there's a
different kind of spectrum as well where
there's bright there's bright emission
on top of a dark background so perhaps
the material is not emitting light at
all all frequencies maybe it's emitting
light at specific frequencies and if
those if the material is emitting it
only at specific frequencies not with a
rainbow but with specific wavelengths of
light then if we pass it through a
spectroscope we see the lines of
emission that that we put here what's
funny we call them lines and that's
simply because when we talk about
emission lines or absorption lines
that's simply because we're using a slit
type opening and so we're actually
seeing the image of the slit so in an
absorption Spectrum so or I'm sorry in
an emission spectrum such as these the
emission spectrum themselves that's the
image of the slit through which the
light is going and so you'll see that
the image of the slit the appearance of
the line is the same for each one it's
just the color is different and the
wavelength is different so that's
interesting and we also notice that
they're that they're are kind of
distinct well emission Spectra are
fascinating because what you can where
you can get them is if you take say
sodium sodium chloride table salt and
you hold it in a bunson burner and you
hold it like a bit of table salt in in
like a metal it's a piece of metal and
you put it over a bunson burner and heat
it up till it glows then the flame that
comes off the bunson burner will have a
particular wavelength and if you look at
the lights that is coming off of the
burning salt that comes off of there you
get a specific set of wavelengths and
those are emission Spectra now you'll
get a sodium spectrum and of course
since it's table saltt you Al going
chlorine Spectrum as well but really we
care about like the sodium Spectrum but
you see that a noble gas such as neon
will have a different spectrum and
mercury vapor has a different spectrum
and ubiquitous hydrogen throughout the
cosmos has has a spe specific Spectrum
so what's funny about all these emission
spectrum is that it doesn't matter where
you get the material from say hydrogen
or helium or neon or sodium chloride or
something or when you get it or from
whom you get it it doesn't matter all
you have to do is heat it up such that
it becomes vaporous and it will emit the
same exact emission spectrum and so
therefore everything has its own
fingerprint hydrogen has the same thing
no matter where you get your hydrogen
from sodium has the same fingerprint no
matter what and we call these things
fingerprints because the emission
Spectra is at exactly the same
wavelengths or frequencies every single
time you measure it which is very
interesting which tells you something
about the nature of the matter so so
what what emission lines can be done and
specifically we're going to be talking
about what are called kof's laws of
spectroscopy and kirov was his full name
is Gustaf kirov and in the mid 19th
century he was doing a whole bunch of
experimental physics and he worked with
the guy who invented the bunson burner
Mr bunson and they did a bunch of
spectroscopy and what he found were the
following Three Laws kof's laws first
say that emission lines are produced at
single frequencies of behind in front of
a dark background if you're looking at a
hot rarified gas that has no bright
emission Source or no bright Continuum
behind it it's just a gas that you see
with nothing bright behind it so you get
specific wavelengths appearing at spe
brightness at specific wavelengths and
that comes from a hot gas and that is
one of cure gloss L of spectroscopy the
next one is if you take a continuous
Source like a light bulb like an
incandescent light bulb which gives a
black body type of radiation and that is
in front and that is behind a cool gas
maybe it's the same gas maybe it's
hydrogen gas that's cooler than the bulb
well as the light from the hot hot bulb
passes through the cooler gas the cool
gas absorbs the light and it and the
absorption then dims the light at
specific frequencies and those spefic
specific frequencies where it gets
dimmed then get passed through we see we
see it as a spectrum and we see
something like the solar
Spectrum but when we look carefully we
actually see something more interesting
deeper in just a bit and so we can
actually relate kof's laws to as a trio
of things because a hot a hot opaque
body such as a light bulb emits a
continuous Spectrum something hot and
opaque emits a continuous Spectrum
something hot an opaque that is behind a
cloud of cooler gas that emits that that
we see if we look through the cooler gas
to the hot Source we will see an
absorption Spectrum now if we look
quarter to that and just look at the gas
that did the absorbing of the light we
will see an emission spectrum so really
Kos laws are are two side are three
sides of the same situation you can
either look straight at the continu
straight at the hot dense object and
you'll see a continuous object
continuous Source you'll look at the uh
Cloud that's sitting off to the side of
the absorption of the hot object and
look through the cloud at the hot body
and you'll see an absorption Spectrum or
you'll look off to the side of just the
emission just the hot Cloud that well
cooler Cloud compared to the hot body
and you'll see an emission spectrum so
they're all part of the same thing and
that means that as we look at these
objects that the emission lines and the
absorption lines are at exactly the same
wavelengths and that makes sense because
the let's say it's a hydrogen Cloud gas
and hydrogen will absorb and emit at
exactly the same wavelengths it's not
like it absorbs at one wavelength and
emits at another no it emits at the same
exact wavelength that it absorbs that's
interesting so it's telling you
something about the nature of matter the
nature of matter itself
and Gustaf kirov did all the work in
spectroscopy well before the Advent of
quantum mechanics and the model of the
the born model of the atom so as we look
at the nature of what matter is he
didn't know about all this stuff so he
derived these laws empirically and these
observations then had to be accounted
for with the Advent of quantum mechanics
which they were so most importantly is
that the setup of the spectroscope tells
us an enormous amount about the nature
of the material so we can TP it depends
on what you how you wish to view things
or what is possible for you to view um
you can set up an experimental apparatus
in many many ways uh in order to do
spectroscopy but it's typically pretty
easy in well not easy typically um it's
helpful to start with an emission
spectrum in order to identify exactly
what material you're looking at and then
hopefully through some chemistry you can
determine really what you're looking at
but spectroscopy itself is considered to
be probably one of the most prevalently
used science device scientific tools at
our disposal to understand exactly what
makes up an uh some kind of object so
kof's laws were were were developed in
the uh early 19 mid 19th century and
refined until the early were not
understood of why they behaved until the
mid 20th century now what can you get
out of spectroscopy well the first thing
is because Every chemical element has
its own particular fingerprint or
signature every single element that must
be present in there must emit light or
absorb light so you can tell the
composition of what in the what's in the
gas that you're absorbing or what the
nature of the cool gas in front of the
hot body is in addition the Spectrum
changes a little bit if you're looking
at ionized gas so if you ionize a gas
meaning the electron gets blown out of
the
and if it's blown out of the atom then
the electric field changes a little bit
which changes the Spectrum just slightly
and so not only do you have to to make
all these fingerprint cataloges of
emission spectrum but you also have to
try to get the emission spectrum of
ionized gases as well which really
becomes kind of a pain because the more
you ionize a heavy element the CH the
actual the Spectrum itself changes
considerably and then if you then take
it and we just talking simple atomic
spectrum Atomic elements if molecules
are involved like such as water or
carbon dioxide or carbon monoxide or
deoxy ribonucleic acid or RNA or
anything if you wish to do spectroscopy
on complex molecules or even simple
molecules every single molecule has its
own individual spectrum and some of them
can be incredibly complex and then we
know that that polyaromatic cyclic
polyaromatic cyclic hydrocarbons are
actually present in space and can be
observed because of their Spectrum in
fact ethyl alcohol has been found in
space and some extraordinarily simple
amino acids have been found in space so
this is really interesting we can know
something about the nature of which
molecules are present not even having to
go there it's really fascinating now the
other thing that's interesting about the
nature of spectroscopy is that because
it takes energy in order to we had a hot
remember we had a hot bulb in front of a
cool gas that hot bulb provides energy
into the gas and that gas then reacts to
it well if the temperature of the gas is
not hot enough then it might not react
or it might not absorb certain we it
might not might not get the wavelengths
of light it needs in or doesn't get the
energy it needs in order to hop up and
down or uh we can also then determine
that by looking at how the uh gas is
absorbing or emitting in different
wavelengths or frequencies we can
actually determine the temperature of
that gas so an I if you have a gas full
of hydrogen and it's in front it's above
a star if that gas is above right above
the star maybe it's the atmosphere of
the star and the atmosphere is cooler
than the star then we might see a big
absorption feature for say hydrogen but
let's say the star is extremely
extremely hot and the electrons are
ionized off of the hydrogen atom then
you won't get any absorption at all even
though hydrogen is present or maybe the
star is is very very very cool and you
don't get any absorption of hydrogen
because the star doesn't have enough
energy to kick the electrons out of the
lower orbits of the of hydrogen into
higher orbits well this could also and
spectr the spectroscopy can tell you
something about the temperature of it if
you then look at the nature of the lines
themselves we can find that sometimes
the lines in are not exactly as narrow
as we think so maybe you're in a
laboratory you measure the frequency of
hydrogen the the emission lines of
hydrogen and then what you do is you
take that hot gas and put it under
extraordinary pressure if it's under
extraordinary pressure you're going to
find that the lines themselves these
emission lines will be broader because
as it's under pressure the atoms the
hydrogen atoms are moving at a higher
speed which means that some some of them
are doler shifted towards you and away
from you from your emission which means
that as the energy goes goes into and
out of the the uh the energ uh as this
Mo as the atoms of hydrogen are moving
fast as they're emitting then that can
Doppler shift it either towards you or
away from you and thus broaden the lines
and that's called pressure broadening
and the same thing happens with density
so if you have a high dense atmosphere
you can get different kinds of
broadening so each of these things can
be studied inside of the spectrum of the
star and different kinds of broadening
or different profiles to the line
meaning not just that the line is like
okay it's totally uh dark and then
I've got this one bright thing like a
spike maybe it's shaped like a like a
like a funnel may you have funnel type
shapes maybe you've got wings and then a
funnel there's all sorts of ways that
the that matter can interact with light
and the pro the profile of the line
meaning exactly how does it vary around
that particular wavelength can tell you
a lot about the nature of the star
itself or whatever you're looking at and
there's an incredible amount of
information that gets carried along with
it and that gives us the physical data
about what's happening at the conditions
of the matter when the light was emitted
and because now we can understand the
elements of such as pressure and density
and temperature and composition now we
can do physics and the physics then
allows us to understand what's really
going on and that's what we that's what
so spectroscopy and the invention of the
spectroscope and the uh the invention
and Gusto's laws as they began
introduced the concept of astrophysics
to astronomy because up until the
concept of
Astro until the concept of of
spectroscopy really took hold we could
never really know anything about what
was going on in the stars all we could
learn is that this thing's moving it's
going across the sky it's bright but we
didn't really understand and the
Spectrum so it took some time before we
said well look if there must be an
absorption Spectrum coming from that
star well stars are very dim so it took
a long time for the technology to come
around such that you could actually make
a spectrum of a star photographically at
least but once we understood the nature
of the spectrum of a star we now then
could transform astronomy into
astrophysics and that allows us to learn
exactly how something is happening even
though it's hundreds of light years away
or thousands of light years away or
millions of light years away because we
trust that all of the physics that
happens here in the laboratory and this
is a really big assumption this is one
of the most important underlying
assumptions of all astrophysics due to
spectroscopy is that since all the laws
of the physics are the same everywhere
in the universe at all times of the
universe's existence meaning from all
the way ago to all the way today that
the laws of physics themselves are the
same
then we can trust spectroscopy because
we only get a messenger of light so
that's a really interesting assumption
that all of light and all of matter is
the same here as it was there and as we
measure the laboratory wavelengths of
hydrogen and then we look at some
distant quazar and that distant quazar
emits light and it absorbs light and
there's absorption of light as it
travels to us through through the
intergalactic medium then we can trust
that we can actually understand what's
happening between us and the quazar and
even at the quazar that emitted the
light say 10 billion years ago and the
light traveled for 10 billion years so
the universality of the laws of physics
is what we trust because we've never
been to the Stars we probably never will
be to the stars but the physics of light
interact with matter allows us to reach
with our imagination out to the stars
and learn about what is happening over
there and we'll see the effect of
spectroscopy everywhere throughout all
of these courses and that's how we
actually know what's going on over there
all right we'll see you next time
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