Light: Crash Course Astronomy #24

By | December 5, 2019


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.

100 thoughts on “Light: Crash Course Astronomy #24

  1. ShadowCat 1442 Post author

    LIGHT IS RADIATION
    "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)."
    -Wikipedia

    Reply
  2. Krizzia DeJesus Post author

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

    Reply
  3. Tim Mosso's Sunglasses Post author

    Great video but no mention of photons? lol.

    Reply
  4. Owen Sechtman Post author

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

    Reply
  5. Automated_Response85 Post author

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

    Reply
  6. Jason Pettet Post author

    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.

    Reply
  7. Khalid Rao Post author

    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.

    Reply
  8. Jessica Stein Post author

    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

    Reply
  9. Qiuyu Shi Post author

    To go for deeper information, Crash Course Physics

    Reply
  10. gustavo juarez Post author

    Just watching crash course videos to study for midterm ahahaha

    Reply
  11. 2008koss Post author

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

    Reply
  12. D nahar Post author

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

    Reply
  13. imaan islam Post author

    this is what they use in flame emission spectroscopy
    very cool

    Reply
  14. Me Post author

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

    Reply
  15. Taylor Barnett Post author

    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

    Reply
  16. david ortega Post author

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

    Reply
  17. Tanish J Post author

    omai… I just noticed the KSP figures on his table

    Reply
  18. Joyce Hayek Post author

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

    Reply
  19. PandaLivesMatter Post author

    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?

    Reply
  20. Chris Stevens Post author

    What property of light explains why eyeglasses help correct vision?

    Reply
  21. Ayushi Kangotra Post author

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

    Reply
  22. Sougata Biswas Post author

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

    Reply
  23. ABU TALHA Post author

    I read books and watch your video whole day.

    Reply
  24. Daniel Scimé Post author

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

    Reply
  25. siva sankar Post author

    Good one .. enlightened me the concept EMR…

    Reply
  26. Rushil Jain Post author

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

    Reply
  27. poppin adi Post author

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

    Reply
  28. CrazyParanoidResearcher and Artist Post author

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

    Reply
  29. Mia Lane Post author

    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.

    Reply
  30. Muturu Imortalz Post author

    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

    Reply
  31. राष्ट्रपिता Post author

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

    Reply
  32. Mat Mat Post author

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

    Reply
  33. M Bayrak Post author

    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.

    Reply
  34. S Km Post author

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

    Reply
  35. Loll Ger. Post author

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

    Reply
  36. NightCrawler1000 Post author

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

    Reply
  37. Nathan M. LAZER Post author

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

    Reply
  38. María Carla Post author

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

    Reply
  39. Yukihyo Post author

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

    Reply
  40. Science Revolution Post author

    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.

    Reply
  41. Le H Is Good Post author

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

    Reply
  42. Chris Casillas Post author

    The wave is really a side view of a spinning spiral

    Reply
  43. Drake H Post author

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

    Reply
  44. shan z Post author

    i love this episode. he's calm and slow and comprehensible!

    Reply
  45. Kim's Travel life diaries Post author

    Thank you for this really enlightening video. I remember doing all these in Physics some 17yrs ago.

    I only stumbled on this channel because my 2year old was holding a CD and she noticed the different colours and asked me …"mummy look, so many colours. What is that?" Then I remembered learning about a spectrum etc.

    Oh my child…you make me learn all sorts.

    Reply
  46. Vektor Post author

    NOTES:
    Most information about the universe comes in the form of light

    Light is an electromagnetic wave and a form of energy

    Wavelength is the distance between the crests

    Shorter the wavelength, higher the energy (tighter wave)

    Shorter the wavelength, higher the frequency

    That's what differentiates colors in the visible spectrum

    Blue, Ultraviolet, x-ray and gamma rays are the shorter ones

    Red, Infrared, microwave and radio waves are the the longer ones

    Heating up a system raises the energy and to get rid of that it might emit light

    Temperature and the light emitted are therefor related

    In atomic structures electrons only get to occupy specific volumes of space

    That depends on their level of energy

    Light hitting the atom can provide the energy required to raise the energy level

    Upon loosing that energy it would emit light

    Different atoms have a different set of energy levels and emit different colors

    That's how we identify the components of far away bodies

    Spectrometer is a device to measure the wavelength

    Doppler effect is the compression and extension of wavelengths caused by movement

    In sound pitch changes, and in light color

    So we can tell the direction of movement of bodies

    The red-shift revealed the expansion of the universe to us

    We also can learn about their spinning, electromagnetic fields and densities

    Reply
  47. Lizard King Post author

    Nothing in nature is exact… electrons must have a +/- tolerance for what photons will cause a jump? What is that tolerance? 0.001% 0.00001% Or do photons have certain allowed wavelengths? eg no gaps between say 500.5nm and 500.5000001nm?

    Reply
  48. Yeoj Also Known As MEGAMIND Post author

    Thank you very much Sir for Learning new things <3

    Reply
  49. Collin Smith Post author

    This series is AWESOME!!! I never knew how interested I was in astronomy until discovering this series. I’m totally blown away by all the great info in this course! Really wish they would’ve taught this interesting stuff in high school. ?

    Reply
  50. Safiah ahmed Post author

    This is the best channel ever. It's amazing what you are doing guys

    Reply
  51. Marcus Chan Post author

    A good way to remember the spectrum is this:Roger Makes Instruments Violins Ukeleles Xylophones & Guitars (Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays & Gamma

    Reply
  52. Bee Yang Post author

    woah…… now i understand those damn space pictures now. AMAZIIIIIIIIIIIIIIIING!

    Reply
  53. Amy Van Gundy [Rancho HS] Post author

    So clear and succinct! I love the host! Slow, clear and engaging!!

    Reply
  54. Ritvik Vaishnav Post author

    i love that into
    teee du du du, duh duh duh, tee duh duh duh

    Reply
  55. Phil Rodriguez Post author

    This is amazing. By figuring out the mechanics of light, humans unlocked the mysteries of the universe. Also, the way light takes time to move, it can show us the past. Once the James Webb Telescope is launched into space by NASA, it will allow us to look back at event in our universe which happened over 13 billion years ago, to the very origins of creation. It truly boggles the mind

    Reply
  56. shachar chalak Post author

    6:44 than can electrons be stripped of their atoms if you were to give them a lot of energy?

    Reply
  57. kok fah chong Post author

    Einstein's famous equation, E=mc^2 is wrong otherwise garbage also can be used to make nuclear bombs as long as it is matter or it has mass. Energy and matter can't interchange one another according to Einstein's famous equation. One must have photons before one can emit out photons. Photons are particles and they have mass. All forms of EMWs including light and heat are dynamic photons per volume per time in different saturations. NASA claimed that Hubble Space Telescope can see the formative state of the young universe which implies that celestial bodies were travelling much faster than the speed of light in the past but somehow they have slowed down for long enough slow enough now for the light of the past to catch up with us that we can see the young universe. This is simple kinematic problem. If the universe is expanding at accelerating rate, then we couldn't see the past of the universe when it was young. Red-shift only tells us that it is expanding. But we can't use Doppler effect equation to gauge the expansion rate of the universe because light is not wave. Generally dimmer stars have higher red-shift readings. Halton Arp discovered that different portions of a galaxy can have different red-shift readings. Therefore we can't base on high red-shift reading to deduce that the universe is expanding at accelerating rates. Different colors of light have different saturations of photons per volume per time. Blue light is more powerful than red light because blue light has higher saturation of photons per volume  per time therefore blue light can cause vanes of a radiometer to turn faster due to higher change in momentum that caused by embedded them with higher number of photons per time. Red light can only manage to cause vanes of a radiometer to turn slowly because there are fewer dynamic photons embedded onto vanes of the radiometer per time. If you are interested in real discoveries, I would recommend you to read my book, The Unification Theory – Volume One and you will be amazed with lots of new, interesting discoveries. In God I trust.

    Reply
  58. Luke Zingg Post author

    Question: If over distance wavelengths are compressed, then would that mean if I was listening to a band and they were spread out, and I was not in the center of all of them, they would sound out of tune?

    Reply
  59. antonio39777 Post author

    This essentially made up 2 days of astronomy I missed. All while watching this in the bathroom

    Reply
  60. M Bayrak Post author

    I've added Turkish subtitles quite a few months ago and I am extremely confident that they're as good as it gets.

    Still not approved it seems. Why? Crash Course, I reccomend this channel a lot to my relatives and all, many of them are not that good English speakers.

    Reply
  61. Dell Schanze Post author

    FALSE!! No our eyes did NOT evolve over "millions of years".

    Reply
  62. A.I. Joh-An Gerrison Bot Post author

    LIGHT: "I Am all the colors of the rainbow."

    Reply
  63. Digicraftmon the Crystal Gem Post author

    I don't understand how to calculate 3.00 x 10^8/6.50 x 10^5; how do I convert the numbers or are the numbers treated as a 3.00 or 3?

    Reply
  64. Richard Rodriguez Post author

    A little bit confused here. I hope you can "shed some light" and I mean it. If light emitted from celestial bodies is blue or red-shifted, and I understand that as the universe is expanding, they mostly are red shifted, how can the band of absorption be determined to know the elements in a star for example, if the received light is already modified (redshifted)? Isn't its wavelength already changed or modified? Kinda spoiling the test? How to reconstruct the sample? Am I getting this wrong? Thank you.

    Reply
  65. Seyram Agudu Post author

    7:35
    Is it weird that I can somehow tell that the box on the left is a darker shade than the box on the right

    Reply
  66. rusty al mercado Post author

    Hi crash course i take a screen shot of one of the video i like to use it now if you will be given me feedback that i am not permitted to use it in social media i will remove it thanks

    Reply

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