Light: Crash Course Astronomy #24

Light: Crash Course Astronomy #24

Hey, Phil Plait here and this is Crash Course
Astronomy. In last week’s episode, I mentioned that nearly all the information we have about
the Universe comes in the form of light. But how does that light get made? What can it
tell us about these astronomical objects? And honestly, what is light? Here’s a hint. Light is a wave. It took centuries of thought
and experiments to figure that out, and to also figure out that, at its most basic, light
is a form of energy. It travels in waves, similar to waves of water in the ocean. Except
with light, the things doing the waving are electric and magnetic fields. Literally—light
is a self-contained little bundle of these two fields, intertwined. That’s why we call
light electromagnetic radiation. The details of this are very complex, but we can make
some pretty good overall observations about light just from thinking of it as a wave. If you’re floating in the ocean, you’ll
move up as a wave passes you, then back down, then back up again when the next wave rolls
by. The distance between these crests in the wave is called the wavelength. Since light
is a wave, it has a wavelength as well, and this may be its single most important feature. That’s
because the energy of light is tied to its wavelength. Light with a shorter wavelength has more energy,
and light with a longer wavelength has less energy. And our eyes have a really convenient
way of detecting these different energies: color! What you think of as the color violet is actually
light hitting your eye that has a short wavelength. Red light has a longer wavelength, about twice
the distance between crests as violet light. All the colors in between—orange, yellow,
green, blue—have intermediate wavelengths. This spread of colors, wavelengths, is called
a spectrum. Over millions of years, our eyes have evolved
to detect the kind of light the Sun emits most strongly. Well, that makes sense; that
makes it easier for us to see! We call this kind of light visible light. But that’s just the narrowest sampling of
all the different wavelengths light can have. If light has a slightly shorter wavelength
than what our eyes can see, it’s invisible to us, but it’s still real. We call that
ultraviolet light. Light with shorter wavelengths than that fall into the X-ray part of the
spectrum, and light waves with the shortest wavelengths of all are called gamma rays. At the other end, light with slightly longer
wavelengths than the reddest color we can see is called infrared light. Light waves
longer than that are called microwaves, and those with the longest wavelengths of all
are called radio waves. These different groups don’t really have hard and fast definitions;
just think of them as general guidelines. But together, we call all of these different
kinds of light the electromagnetic or EM spectrum. And remember, energy goes up when the wavelength
gets shorter. So ultraviolet light has a higher energy than violet, X-rays have a higher energy
than that, and gamma rays have the highest energy of all. Infrared light has lower energy
than red light, microwaves lower than that, and radio waves have the lowest energy. When you look at the whole EM spectrum, you’ll
probably notice that we really do only see a teeny little sliver of it. Most of the Universe
is invisible to our eyes! That’s why we build different kinds of telescopes — to
detect the kind of light our eyes can’t detect. They let us see a lot of stuff
that otherwise we’d never notice. So you might be asking: how is light made?
Well, one of the most basic properties of matter is that when you heat it up it gains
energy, and then it tries to get rid of that energy. Since light is energy, one way to
get rid of energy is to emit light. Another important property of matter is that
the kind of light an object emits depends on its temperature. An object that’s hotter
will emit light with a higher energy, that is, a shorter wavelength. Cooler objects give
off light with a longer wavelength. You may have seen this in action. Heat up
an iron bar and it starts to glow red, then orange, then yellow as it gets hotter. The color, the wavelength,
of light emitted changes as the bar heats up. Astronomers use a shorthand for this. We say
that light with a shorter wavelength is “bluer”, and light with a longer wavelength is “redder”.
Don’t take this literally! We don’t really mean more blue or more red, just that the
wavelengths are decreasing or increasing. So in this lingo, ultraviolet light is bluer
than blue, and X-rays are bluer than ultraviolet. So objects that are more energetic, that have a
higher temperature, are bluer than cooler, redder objects. This rule of thumb works really
well for dense objects like iron bars and stars. Even humans! You emit light, but it’s in the
far infrared, well beyond what our eyes can see. There are less dense objects in space, too,
like gas clouds, and the way they emit light is different. To understand that, we have to zoom in on
them. Way, way in, and look at their individual atoms. And to understand that, we need to take a
brief diversion into atomic structure. Atoms are the building blocks of matter. In general,
atoms are made up of three subatomic particles: Protons, neutrons, and electrons. Protons
have a positive electric charge, electrons a negative charge, and neutrons are neutral.
Protons and neutrons are much more massive than electrons, and occupy the centers of
atoms, in what’s called the nucleus. Electrons whiz around the nucleus, their negative charge
attracted by the protons’ positive charge. The type of atom depends on how many protons
it has in the nucleus. Hydrogen has one proton, helium two, lithium three, and so on up the
periodic table of elements. It’s common to think of the electron as
orbiting the nucleus like a planet orbits the Sun, but that’s not really the case.
The real situation is fiendishly complex and involves pretty hairy quantum mechanics, but
in the end, the electron is only allowed to occupy very specific volumes of space around the
nucleus, and those depend on the electron’s energy. Think of these like stairs on a staircase,
where the landing is the nucleus. When you walk up the stairs, you have to use energy
to go up. And when you do, you have to go up a whole step at a time; if you don’t
have the energy to get to the next step, you can’t move. You can be on the first step,
or the second step, but you can’t be on the first-and-a-halfths step. There isn’t
one! Electrons are the same way. They whiz around
the nucleus with a very discrete amount of energy. If you give them an additional precise
amount of energy, they’ll move up to the next energy level, the next step, but if you give
them the wrong amount they’ll just sit there. The opposite is true as well; electrons can
be in a higher energy state, up on a higher step, and then give off energy when they jump
down. The amount they give off is exactly the same amount needed to get them
to jump up in the first place. How do they get this energy? Light! If light hitting the atom has just the right amount
of energy, the electron will absorb it and jump up. It can also jump down and emit light at
that energy, too. An electron can also jump two steps, or three, or whatever, but it needs
exactly the right energy to do it. But as I said earlier, energy and wavelength
are the same thing, and that’s equivalent to color. So when an electron jumps up or down,
it absorbs or emits a very specific color of light. Not only that, but the steps are different
for different atoms. To stick with our analogy, it’s like different atoms are different
staircases, with different heights between the steps. So when an electron jumps down
a step in a hydrogen atom, it emits a different energy, a different color of light, than an electron
jumping down in a helium or calcium atom. And this, THIS, is the key to the Universe.
Because different atoms emit different colors of light, if we can measure that light, in
principle we can determine what an object is made of, even if we can’t touch it. Even if
it’s a bazillion light years away! And we can. Can you tell the difference between these
two squares? They’re a very slightly different shade of red. Your eye probably can’t tell
the difference, but a spectrometer can. This is a device that can precisely measure
the wavelength of light, and can for example distinguish light emitted by a hydrogen atom
from light emitted by helium. When you hook one of these spectrometers up to a telescope, you can
figure out what astronomical objects are made of. In the case of thin gas clouds in space, the
atoms are basically floating free, rarely bumping into one another. The atoms emit those
individual colors of light, allowing us to identify them. Unlike dense stars, the color of the thinner gas
depends more on what’s in it than its temperature. And this is how we learned what the Universe
is made of. Stars and gas clouds in space are mostly hydrogen, with some helium and
heavier elements thrown in. Jupiter has methane in its atmosphere, Venus carbon dioxide. Everything
in the Universe has its own mix of ingredients, like cakes at a bakery. With spectroscopy,
we can taste them. But wait! There’s more. You’re probably familiar with the Doppler
effect; the change in pitch when, say, a motorcycle goes by. In sound, the wavelength defines
the pitch; higher tones (“eeeee”) have shorter wavelengths, and lower tones (“eeeee”)
longer wavelengths. When the motorcycle is headed toward you, the sound waves get compressed,
causing the pitch to rise. After it passes you, the pitch drops because the wavelengths
get stretched out. The same thing happens with light. If an object
is headed toward you, the wavelength of light from the source gets compressed, shorter.
We say the light is blue-shifted. If it heads away, the wavelength gets longer, and it’s
red-shifted. Apply that to a spectrum, and by measuring that shift we can tell if an
object is moving toward or away from us. Here’s a teaser: This becomes super important
later, when we talk about galaxies. Spoiler alert: The Universe is expanding, and it’s
this redshift that allowed us to figure that out. And that’s still not the end of it. With
other spectroscopic techniques we can determine if an object is spinning and how fast, whether
it has a magnetic field and how strong it is, and even how massive and dense an object
is. A vast amount of the fundamental properties of astronomical objects can be found just
by dissecting their light into individual colors. Almost everything we know about the Universe
comes from the light objects in it give off. Pictures of astronomical objects show us their
structure, their beauty, and hint at their history. But with spectra, we can examine
their blueprints. Today you learned that light is a form of
energy. Its wavelength tells us its energy and color. Spectroscopy allows us to analyze
those colors and determine an object’s temperature, density, spin, motion, and chemical composition. Crash Course is produced in association with
PBS Digital Studios. Head over to their channel for even more awesome videos. This episode
was written by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant
is Dr. Michelle Thaller. It was directed by Nicholas Jenkins, the script supervisor and
editor is Nicole Sweeney, the sound designer was Michael Aranda, and the graphics team
is Thought Café.


  • titas totas says:

    Does the light get red shifted by the expancion of space or by or objects moving away? Im quite sure light does not get efected by the emiter moving.

  • VO1D-_-S3CR3TS The Gamer says:

    Thanks for informing me Chief

  • Ragnhild Larsen Øverli says:

    i have a test on this tomorrow so welp here goes nothing

  • Benny Kleykens says:

    light is a wave … so …what is the color of a single photon? If a single photon moves in a wave motion then it still would have a wave lenght and height but it would only hit the eye once , so the eye would never be able to detect the 'lenght' or 'height' of the movement of that photon?!

  • Daniel Hain says:

    Man for anyone who disliked this video please go Dye 🙂 Like in food coloring

  • Michelle O'Brien says:

    I'm french, and I understand better the crashcourse's videos that my lessons in french at high school. Thank you! 🙂

  • Azerty Keys says:

    8:43 John Green!!!!

  • מירב ברוכים says:

    thans for the perfect video
    can you explain please about the kirchhoff thee lows of spectroscopy

  • Vishal Upadhyay says:

    "The Universe is expanding." gave me chills.

  • Channel 123 PLEASE DO NOT SUBSCRIBE 567 says:

    And I thought this would help me make better artworks…

  • Razzola P says:

    Dumb and dumber bike with Phil and hank holding Mary Swanson’s briefcase lol

  • Ayaan Sabir Ahmed says:

    I understand everything about EM waves! But the thing with which i am having a tough time is getting my head around the physical nature of the movement of light. Is it like the photons move up and down like a wave? EM waves have waves. But what are waves physically? How do they pass through everything and go anywhere?

  • M R says:

    Thank you so much, this is just what I needed! definitely learned a lot.

  • Sarah Ali says:

    Wow, thank you!

  • RobertMOdell says:

    CrashCourse. How many "wave cycles" are there in a single photon ?

  • isse roble says:

    light a t a 2d record is a wave but observed in three dimensions is a reaching swelling bubble that pushes off from the energy source in all directions and exerts a push on itself which is recorded as the anti node. light faces no intertia and there fore can spread at no inertia. I think inertia is different points on the axis. High inertia needs more energy low needs less yada yada mass is caused by inertia. I understand energy and now need to understand inertia or mass which I think is a product of expansion. such that the two forces electro and magnetic balance out to 0 with light and with other energy electro might have an advantage and with still others the magnetic force might have the advantage. like the sun is a massive light with electro that is larger than the earth and magneto that is larger than the earth and fully expressed both are to the scale of the solar hold. such that we are a real reaction between the sun's electromagnetism and the earth's elemental construction. so there's the origins of life for you. you're welcome. I am annoyed by your hesitation.

  • Achaeos Salisbury says:

    So can you construct a device which will detect a specific element for you?
    Like, what if you needed iron, could you create sun glasses which only allow the light from iron to reach your eyes?

  • Canadian Man says:

    What if you have ultra frequency with near inexistent amplitude? Call it a neutrino?

  • Yitz Chalfari says:

    Who dislikes an informative video like this I'm confused.

  • Jean Kathleen says:

    Woah. This is amazing.

  • Reymark Cabanacan says:

    He is best

  • Ian Wilson says:

    Very good information enjoyed. This guy is excellent at conveying very complex information in more easily stood terms

  • hotchili b says:

    Thank you so much omg

  • Rene Kennicutt says:

    The left red square is slightly darker/grayer than the right square. Can anyone else see this, or is it just my mind playing tricks on me? He says you can't tell the difference and now I'm going crazy bc I did.

  • Ariel Currá says:

    This episode was awesome

  • Shreeya Mittal says:

    5:15 this periodic table is just so cute!

  • Genius Boss says:

    Who else figured out that the first part of the intro said “a giant leap for mankind”

  • nounou chan says:

    This is fxckin cool! I learned so much just from this 10 minute video!

  • Siddharth Buddha says:


  • Pushpa Rani says:

    Is that hank in the scooter 😂🤣

  • Amy Barrera says:

    I really appreciate the passion and excitement of this video!

  • Sparklin Waffles Tubin' says:

    How can you tell if an object far away is either red shifted or emitting energy as red light?

  • Knight Ace says:

    What about photons

  • Marcus Webb says:

    So what type of Electro Magnetic Punch or Wavelength is white light packing then If its the combination of all the other colors of visible light….
    Far out bro

  • Oz Cohen says:

    this guy is amazing. i should start crashcourse astronomy even though im not studying anything envolves with astronomy.
    his just brilliant.


    Is there any distinction between light(energy) and matter(energy)? Or everything falls into one identity?

  • Simon Drouart says:

    Thanks !
    Was super genial!

  • James Roy says:

    How come I can’t ever watch a science video without being repeatedly and randomly reminded that millions of years ago we evolved in “such and such a way” so very fortunately for our survival…??? Any independent thinking, unbiased person could easily see people like this are trying too hard. I’d respect their theories more if they weren’t always compelled to just throw in their propaganda with assumptive claims of “fact” about a “theory” that has TONS OF HOLES and that really doesn’t add meat to the subject they are explaining… leave your agenda out of it. Evolution and the Big Bang is not settled science… it’s just made to seem that way by atheists who are as loud and obnoxious as the religionists they hate so much. But their just like them. Today’s so called “science” is just a pagan religion hiding behind half truths.

    Insert salty replies here 👇🏼.

  • Charles Hancock says:

    Give this man a like

  • Mazen Ali says:

    I like this show

  • Madison Norton says:

    Cool vid

  • Rob Lindsey-Nassif says:

    These videos are really a gift to the world. Phil is brilliant, articulate, and fun.

  • ヨーソローちゃん_17 says:

    That question never came across my mind, "how is light made?"

  • MvpMartell says:

    Light is a particle and a wave

  • Mr. Coffee says:

    So light is just pure energy of different level ?! And it has no mass ?!

    Why are we burning oil then !? Lets just gather light energy wich is abundant and infinite !!

  • Kanishk Rawat says:

    Sir I get a wide vision about light

    Truly very interesting 😘😘😘

  • Michelle Koh says:

    Was not prepared for the cuteness of the periodic table. Elatedly surprised!

  • D - Boss says:

    7:35 Who actually saw the difference? It's actually not that hard…

  • mklik4 says:

    This is the best video on YouTube for this subject, and I've watched many. Thanks!!

  • hotdrippyglass says:


  • ShadowCat 1442 says:

    "Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. … Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths)."

  • Krizzia DeJesus says:

    thank u so much mr plait. i felt like i was dying. sincerely, a college student taking astronomy 130

  • Tim Mosso's Sunglasses says:

    Great video but no mention of photons? lol.

  • Owen Sechtman says:

    Ive been watching alot of Science videos lately, my dream is to become an astrophysicist.

  • Automated_Response85 says:

    Ughh.. Calculating the shift in energy states in general chemistry 2 was NOT fun…

  • Jason Pettet says:

    A miss application of the Doppler Effects has lead to the belief the universe is expanding.
    The 'Red Shift' only proves the light wave is lengthening. That can be due to Slowing of Light also. Something that doesn't violate the Law of Thermodynamics, as does the current theory of an expanding universe accelerating.
    Slowing light speed would account for the larger shift in speed the farther you look back.

  • Khalid Rao says:

    Its really amazing . This lecture has proved very useful in clearing my concepts and many doubts as well. I really felt educated on the subject. Thanks a lot and stay blessed.

  • Jessica Stein says:

    I'm studying for my Intro to Astronomy final exam, and I would estimate that you just covered 6 weeks of our curriculum within 12 minutes. Color me impressed, this was truly enlightening and you are making waves my good sir. Although I still would rather have my molecules scrambled by gamma rays than take my final. Don't let Marvel fool you btw, gamma rays don't turn you into the Hulk. Try radiation sickness or possible incineration

  • Qiuyu Shi says:

    To go for deeper information, Crash Course Physics

  • dnjoncvichy says:

    8:45 Vsauce on roadside

  • gustavo juarez says:

    Just watching crash course videos to study for midterm ahahaha

  • 2008koss says:

    it was a great course, but I didn't hear the term "photon" even once !?

  • D nahar says:

    Talking about colours???
    Hubble generally captures black and white(for better exposure) pics

  • imaan islam says:

    this is what they use in flame emission spectroscopy
    very cool

  • Me says:

    Best video ever !!! I wish you were my professor !

  • Taylor Barnett says:

    Wait if Red is lower energy then why do things turn red or yellow when they are hot ? What? You just said Red is less energy and purple is hotter

  • david ortega says:

    so the humanity can know staing at home what it is made of things billion light years away, we are amazing c:

  • Tanish Janardhanan says:

    omai… I just noticed the KSP figures on his table

  • Joyce Hayek says:

    I spent a whole year trying to learn these, why didn't I come here? Thank you for this!

  • PandaLivesMatter says:

    We can see the color transition on a visible light spectrum, but is there any way we can see visible light wave transitioning into infra-red or ultra violet light?

  • Chris Stevens says:

    What property of light explains why eyeglasses help correct vision?

  • Ayushi Kangotra says:

    Watched this at 2am here, who knew science could be this interesting 😂
    Thanks to Phil! And cc.

  • Sougata Biswas says:

    9:15 if its expanding then what is around it?

  • ABU TALHA says:

    I read books and watch your video whole day.

  • Daniel Scimé says:

    I thought our eyes evolved to be sensitive to the part of the spectrum that best penetrated water?

  • siva sankar says:

    Good one .. enlightened me the concept EMR…

  • Rushil Jain says:

    I don't think humans emit light, they reflect lights of the stars!?

  • Ace H says:

    This was cool as hell

  • poppin adi says:

    Sir According to you we emit ifrared rays then we exhibit bioluminiscence ..right so basically humans are ctenophores….😂😂😂😂

  • CrazyParanoidResearcher and Artist says:

    I see this guy everyday on How The Universe Works. I never thought I would see him on youtube like this.

  • miajolane says:

    This is a great way to explain emission spectrometry for high-school chem! Helps you to understand why the electrons give off light, energy levels, the quantum (discrete) nature of these levels and wavelength without having to bamboozle you with quantum physics.

  • Muturu Imortalz says:

    oh yee! it is not every day that you have to stop a video halfway to like and subscribe to the channel… this is amazingly done

  • Tushar says:

    Thanks for the efforts of guys you have mentioned at the end❣️

  • Mat Mat says:

    If light is a wave, what is darkness? I mean, darkness is the absence of light, but…. can we consider darkness a different wave ?

  • M Bayrak says:

    I'm rewatching these series. And I just remembered how eye opening about science this particular episode is. Very, very great one. So much so, I've spent my last 2 hours, maticilously translating it to a very good Turkish. People should watch this in every possible language.

  • s km says:

    I love this show. Really, I do. And Astronomy is just sooooooo beautiful !!!!!

  • Loll Ger. says:

    Anyone else think the two red squares were quite different shades?

  • NightCrawler1000 says:

    Great video, helped me in understanding the properties of light 👌👌

  • Nathan M. LAZER says:

    Imma use the blue and red shift concept to show distancing in my art now. Thx.

  • lar B says:

    why do they all takl like we are 2 year olds

  • María Carla says:

    hey guys ♡ i made a whole video explaining the electromagnetic spectrum with ariana grande's discography 🙂

  • nano tech says:

    Loved it

  • Yukihyo says:

    The left square looked darker to me, or is that just my imagination?

  • Science Revolution says:

    Light is visible alternating current.

    Lightwaves must exist within matter.

    Lightwaves are longitudinal waves.

    Light is not a photon particle.

    Light cannot become matter.

    Light is the universe's free electricity.

    Sunlight stays in Sun's plasma.

    Sunlight on earth stays in Sun facing atmosphere.

    How does sunlight travel through space to reach us?

    Sunlight stays within the plasma on the Sun; sunlight does not travel in space.

    Sunlight transfers its energy to earth instantly through an invisible giant rigid rod.

    One end of the rod sets on the Sun's plasma and covers half of the Sun, while one end sets on earth's atmosphere and covers half of the earth.

    That rod is made of the repulsion force between the line of sight electrons on the surface of the

    Sun's plasma and on the surface of the earth's outer atmosphere.

    That rod of force is, in fact, billions of times stronger than a steel rod; it locks the atoms touched on both ends as one, conducts electrical resonance between the Sun and the earth, and keeps alternating current on the Sun and on earth oscillating at the same frequency.

    The same rod of repulsion force connected all the stars, moons, planets and every mass in the universe altogether, just like gravity connected them all together. Gravity pulls all matters together, while repulsion force pushes all matter apart.

    Coulomb discovered repulsion force and attraction forces are like the hands of God. God holds the universe with his left hand and plays a love song with his right hand.

  • Le H Is Good says:

    Glad white people are liars now they can be responsible for the world collapsing

  • Chris Casillas says:

    The wave is really a side view of a spinning spiral

  • Drake H says:

    I've never seen a video that explains this more beautifully. I just wish he had went more in depth on photons.

  • Moveen Binuwara says:

    The right one is slightly darker

  • Gjrea Edwin says:

    The left one was a paler red

  • Narasimha Chopparapu says:


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