Light: Crash Course Astronomy #24

Summary

This episode of Crash Course Astronomy explores the nature of light, revealing it as a form of energy with wavelengths that determine its color and energy. The video explains the electromagnetic spectrum, the relationship between light, color, and energy, and how spectroscopy allows us to analyze light to determine the composition and properties of astronomical objects. It also touches on the Doppler effect's role in understanding the motion of celestial bodies and the expansion of the Universe.

Takeaways

• 🌌 Light is a form of energy and travels in waves, with electric and magnetic fields oscillating in a self-contained bundle, known as electromagnetic radiation.
• 🌈 The wavelength of light is its most important feature, as it is directly tied to the energy of the light; shorter wavelengths correspond to higher energy and longer wavelengths to lower energy.
• 👀 Human eyes can detect a narrow range of wavelengths known as visible light, with violet at the short wavelength end and red at the long wavelength end.
• 🔬 Beyond visible light, there are other types of electromagnetic radiation such as ultraviolet, X-rays, gamma rays, infrared, microwaves, and radio waves, each with their unique wavelengths and energies.
• 🔭 Telescopes are built to detect different types of light that are not visible to the human eye, allowing us to observe more of the universe.
• 🔥 When matter is heated, it gains energy and emits light as a way to get rid of that energy, with the type of light emitted depending on the object's temperature.
• 🌡 Objects with higher temperatures emit light with shorter wavelengths ('bluer' light), while cooler objects emit longer wavelengths ('redder' light).
• ⚛ Atoms, composed of protons, neutrons, and electrons, are the building blocks of matter, with electrons occupying specific energy levels around the nucleus.
• 🚀 Electrons can absorb or emit light when they jump between energy levels, with the specific color of light corresponding to the energy difference between levels.
• 🧬 Different atoms emit different colors of light when their electrons transition between levels, allowing us to determine the composition of astronomical objects through spectroscopy.
• 🚀 The Doppler effect can be observed in light as well, with wavelengths being compressed (blue-shifted) when an object is moving towards us and stretched (red-shifted) when moving away, providing information about the motion of celestial bodies.

Q & A

What is the fundamental nature of light?

Light is a form of energy that travels in waves, with electric and magnetic fields oscillating in phase with each other. This is why it is called electromagnetic radiation.

How is the energy of light related to its wavelength?

The energy of light is inversely related to its wavelength. Light with a shorter wavelength has more energy, while light with a longer wavelength has less energy.

What is the significance of the visible light spectrum to human vision?

Visible light is the narrow range of wavelengths to which the human eye is sensitive. It allows us to perceive color, which is a result of different wavelengths interacting with our eyes.

What are the different types of light beyond the visible spectrum, and how are they categorized?

Beyond the visible spectrum, light can be categorized into ultraviolet light, X-rays, gamma rays (all with shorter wavelengths and higher energy), and infrared light, microwaves, and radio waves (all with longer wavelengths and lower energy). These are collectively known as the electromagnetic spectrum.

How do telescopes help us observe the universe beyond the visible light spectrum?

Telescopes equipped with sensors and filters can detect various types of light beyond the visible spectrum, allowing us to observe astronomical phenomena that are otherwise invisible to the human eye.

What is the basic principle behind the emission of light from heated matter?

When matter is heated, it gains energy and seeks to release it. One way it does this is by emitting light, which is a form of energy. The type of light emitted depends on the object's temperature.

How does the color of light emitted by an object relate to its temperature?

The color of light emitted by an object changes with its temperature. Hotter objects emit light with shorter wavelengths ('bluer' light), while cooler objects emit light with longer wavelengths ('redder' light).

What is atomic structure, and how does it relate to the emission or absorption of light?

Atoms consist of protons, neutrons, and electrons. Electrons can occupy specific energy levels around the nucleus. When they absorb or emit energy in the form of light, they jump between these levels, resulting in the emission or absorption of light at specific wavelengths.

How does spectroscopy enable us to determine the composition of astronomical objects?

Spectroscopic analysis measures the wavelengths of light emitted or absorbed by atoms. Since different atoms emit or absorb light at specific wavelengths, by analyzing these wavelengths, we can determine the chemical composition of astronomical objects.

What is the Doppler effect, and how does it apply to light in astronomy?

The Doppler effect is the change in frequency (and wavelength) of a wave in relation to an observer moving relative to the source. In astronomy, it is observed as a blue shift (shorter wavelength) when an object is moving towards us and a red shift (longer wavelength) when it is moving away, allowing us to determine the motion of celestial objects.

How can spectroscopy reveal additional properties of astronomical objects beyond composition?

Advanced spectroscopic techniques can reveal properties such as the spin, motion, presence of a magnetic field, and the mass and density of astronomical objects by analyzing the shifts and patterns in their emitted light.

Outlines

00:00

🌌 Light and the Electromagnetic Spectrum

This paragraph introduces the concept of light as a form of energy and a wave, specifically an electromagnetic wave. It explains that light is made up of electric and magnetic fields and travels in waves with varying wavelengths. The energy of light is directly related to its wavelength, with shorter wavelengths corresponding to higher energy and longer wavelengths to lower energy. The visible light spectrum is just a small part of the entire electromagnetic spectrum, which also includes ultraviolet, X-rays, gamma rays, infrared, microwaves, and radio waves. The paragraph emphasizes the importance of wavelength in determining the color of light and how different colors represent different energies detectable by the human eye.

05:04

🔬 Atomic Structure and Light Emission

The second paragraph delves into the atomic structure, focusing on the roles of protons, neutrons, and electrons within an atom. It describes electrons as occupying discrete energy levels and explains how they absorb or emit light when transitioning between these levels. The paragraph highlights the relationship between the energy levels of electrons and the color of light emitted, which is unique to each element due to the specific energy differences between levels. This principle is fundamental to spectroscopy, allowing scientists to determine the composition of distant astronomical objects by analyzing the light they emit.

10:08

📊 Spectroscopy and the Analysis of Light

This paragraph discusses the use of spectroscopy to analyze light and determine the properties of astronomical objects. It explains how a spectrometer can measure the precise wavelengths of light, distinguishing between the light emitted by different atoms. The paragraph also touches on the Doppler effect as it applies to light, allowing astronomers to determine the motion of objects in space. Furthermore, it mentions other spectroscopic techniques that can reveal an object's spin, magnetic field strength, mass, and density. The paragraph concludes by emphasizing the importance of light in understanding the fundamental properties of the Universe.

Keywords

💡Light

Light is a fundamental concept in the video, defined as a form of energy that travels in waves. It is crucial for understanding the theme of the video, as it explains how light's properties, such as wavelength and energy, are key to interpreting astronomical observations. For example, the script mentions that light is made of electric and magnetic fields and is referred to as electromagnetic radiation.

💡Wavelength

Wavelength is a critical term in the script, denoting the distance between successive crests of a wave, including light. It is essential because the energy of light is directly related to its wavelength, with shorter wavelengths corresponding to higher energy and longer wavelengths to lower energy. The script uses the concept of wavelength to explain the visible spectrum of light and how different colors relate to different wavelengths.

💡Electromagnetic Radiation

Electromagnetic radiation is a broad term encompassing all types of light, from radio waves to gamma rays. The script explains that light is a self-contained bundle of electric and magnetic fields, hence the term 'electromagnetic.' This concept is central to understanding the diverse types of light beyond the visible spectrum that astronomers study.

💡Visible Light

Visible light is the narrow range of the electromagnetic spectrum that human eyes can detect. The script emphasizes that our eyes have evolved to be particularly sensitive to the light emitted most strongly by the Sun, which falls within this range. Visible light is a subset of the broader electromagnetic spectrum and is used as a reference point to discuss other types of light.

💡Spectrum

A spectrum, in the context of the video, refers to the range of colors and corresponding wavelengths that light can have. The script mentions the visible spectrum, which is the portion of the electromagnetic spectrum that our eyes can perceive, and uses it to illustrate how different colors relate to different wavelengths.

💡Ultraviolet Light

Ultraviolet light is a type of electromagnetic radiation with wavelengths shorter than visible violet light but longer than X-rays. The script explains that ultraviolet light has higher energy than visible light and is beyond the range of human vision, illustrating the concept of how different types of light exist beyond what we can see.

💡Infrared Light

Infrared light is electromagnetic radiation with wavelengths longer than the red light that humans can see. The script uses infrared light to illustrate how light with longer wavelengths has lower energy than visible light and how different objects, including humans, emit infrared light.

💡Spectrometry

Spectrometry is the process of measuring and analyzing the spectrum of light emitted or absorbed by an object. The script highlights the use of spectrometers to determine the chemical composition of astronomical objects by analyzing the specific colors, or wavelengths, of light they emit or absorb.

💡Doppler Effect

The Doppler effect is a phenomenon where the frequency of a wave changes due to the relative motion between the source and the observer. In the context of the video, the Doppler effect is applied to light, where the script explains how the shift in wavelength (towards blue or red) can indicate whether an object is moving towards or away from the observer.

💡Electron

Electrons are subatomic particles that orbit the nucleus of an atom and play a key role in the script's discussion of atomic structure and light emission. The video describes how electrons absorb or emit energy in the form of light when they jump between energy levels, which is fundamental to understanding how different atoms produce different colors of light.

💡Spectroscopic Techniques

Spectroscopic techniques refer to the methods used to analyze light using spectroscopy. The script mentions these techniques as a way to determine various properties of astronomical objects, such as temperature, density, spin, motion, and chemical composition, by examining the light they emit or absorb.

Highlights

Light is a form of energy that travels in waves, with electric and magnetic fields as the 'waving' components.

Light is known as electromagnetic radiation due to its intertwined electric and magnetic fields.

Wavelength is a key characteristic of light, with shorter wavelengths correlating to higher energy.

The human eye perceives different light wavelengths as colors, with violet having the shortest and red the longest wavelength.

The visible light spectrum is just a small part of the entire electromagnetic spectrum.

Beyond visible light, the electromagnetic spectrum includes ultraviolet, X-rays, gamma rays, infrared, microwaves, and radio waves.

Telescopes are built to detect types of light beyond the visible spectrum, revealing more about the universe.

Heating matter causes it to emit light as a means of releasing energy.

The color of light emitted by an object changes with its temperature, with hotter objects emitting 'bluer' and cooler 'redder' light.

Atoms are composed of protons, neutrons, and electrons, with electrons occupying discrete energy levels around the nucleus.

Electrons absorb or emit specific colors of light when they jump between energy levels.

Different atoms emit different colors of light, allowing for the identification of elements through spectroscopy.

Spectrometry can distinguish between light emitted by different elements, even at vast distances.

The Doppler effect in light allows astronomers to determine if an object is moving towards or away from us based on wavelength shifts.

Spectroscopic techniques can reveal properties such as spin, magnetic fields, mass, and density of astronomical objects.

Almost all knowledge about the universe comes from the analysis of light emitted by celestial objects.

Spectra provide a detailed blueprint of astronomical objects, beyond what is visible to the naked eye.

Transcripts

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.