Kirchhoff's Laws of Spectroscopy

Summary

In 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

• 🌟 Spectroscopy is the study of how light interacts with matter and how this interaction is transmitted through space.
• 🔬 By breaking down light into a spectrum, we can analyze the intensity of light varying with wavelength or frequency.
• 🌈 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.
• 📊 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.
• 🌞 The Sun's spectrum is an absorption spectrum, showing light absorbed from the continuous background.
• 💡 Emission spectra occur when a material emits light at specific frequencies, creating bright lines on a dark background.
• 🔋 Elements emit specific emission spectra when heated, regardless of their source, providing a unique fingerprint for identification.
• 🔬 Kirchhoff's laws of spectroscopy describe the relationship between emission and absorption spectra and the conditions under which they occur.
• 🌡 Spectroscopy can reveal the temperature of a gas by how it absorbs or emits light at different wavelengths or frequencies.
• 🌌 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.
• 🚀 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

00:00

🌌 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.

05:03

🔬 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.

10:04

🔍 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.

15:05

🌟 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.

Keywords

💡Spectroscopy

Spectroscopy is the study of the interaction between light and matter. It involves analyzing how light is absorbed, emitted, or scattered by matter, which can reveal the composition and physical properties of the substance. In the video, spectroscopy is central to understanding how astronomers can study celestial objects by analyzing the light they emit or absorb. The lecturer explains that by breaking light into a spectrum, scientists can observe how the intensity of light varies with wavelength or frequency.

💡Prism

A prism is a transparent object, often made of glass, that can refract light to disperse it into its constituent colors, creating a spectrum. In the script, the lecturer uses the prism as an example of how white light can be dispersed into a rainbow effect, which is a simple demonstration of spectroscopy. The prism is used to show how different wavelengths of light are bent at slightly different angles, resulting in a spectrum.

💡Spectrum

A spectrum is the band of colors produced when light is dispersed by a prism or other device. It is a key concept in spectroscopy as it represents the distribution of light intensity across different wavelengths or frequencies. The video discusses how a spectrum can be a continuous range of colors (continuum) or show dark lines (absorption lines), which indicate wavelengths of light that have been absorbed by a substance.

💡Absorption Lines

Absorption lines are dark lines seen in a spectrum that indicate specific wavelengths of light that have been absorbed by a substance. The lecturer explains that when sunlight passes through a slit and then a prism, the resulting spectrum shows a continuous rainbow with darker spots, or absorption lines, where light has been absorbed by elements in the sun's atmosphere.

💡Emission Lines

Emission lines are bright lines seen against a dark background in a spectrum, indicating specific wavelengths of light that are emitted by a substance. The video describes how heated elements, like sodium in table salt, emit light at specific wavelengths, creating characteristic emission lines. These lines are like fingerprints, as each element emits light at the same wavelengths every time.

💡Continuum

The continuum in spectroscopy refers to the continuous spectrum of light that appears without any interruptions from absorption or emission lines. The lecturer mentions that the rainbow effect seen when sunlight passes through a prism is an example of a continuum, where the light is not absorbed or emitted at specific wavelengths, but rather appears as a smooth band of colors.

💡Kirchhoff's Laws

Kirchhoff's Laws are three empirical laws that describe the relationship between emission and absorption spectra. The video explains these laws as follows: a hot, opaque body emits a continuous spectrum; a hot body with cooler gas in front of it will show an absorption spectrum; and a cool gas will show an emission spectrum. These laws are fundamental to understanding how spectroscopy can reveal the composition and physical conditions of celestial objects.

💡Fingerprint

In the context of the video, 'fingerprint' refers to the unique emission spectrum of an element, which is consistent regardless of the source of the element. This concept is used to explain how scientists can identify elements by their characteristic emission lines, similar to how a unique fingerprint can identify a person.

💡Ionized Gas

Ionized gas is a gas in which some atoms have lost one or more electrons. The video mentions that the spectrum of an ionized gas can be slightly different from that of a neutral gas, which means that the way it absorbs or emits light changes. This is important for spectroscopists to understand when analyzing the composition of celestial objects.

💡Doppler Shift

The Doppler Shift is a change in the frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. In spectroscopy, this effect can cause emission lines to appear broader or narrower, depending on whether the source is moving towards or away from the observer. The lecturer explains that this can provide information about the motion of gases in space.

💡Astrophysical

Astrophysical refers to the application of physics to understand astronomical phenomena. The video emphasizes how spectroscopy has transformed astronomy into astrophysics by allowing scientists to analyze the composition, temperature, and motion of celestial objects based on the light they emit or absorb.

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