Video Transcript: The Electromagnetic Spectrum

So let's do that.  That's what this video is about; trying to understand the fundamental basics of, of light. 

Now, what's awesome is that the light that we see has, in a sense, hidden information with inside of it.  We can see this in in typical rainbow.  When light is passed through a prism, that light is spread out, so what we perceive normally as white light is actually many, many colors of light that have all been combined together.  So this is the first indicator that something deeper is going on with light.  When I look at something and it looks like a white light bulb, or it looks like a green leaf, there's more there than meets the eye.  Now that notion is not new, I mean, people have seen rainbows since the time of Noah, but understanding that white light and the rainbow of light are really the same thing really only goes back maybe 500 years.  And our understanding goes even deeper though with more recent experiments.

The well-known physicist William Herschel devised an experiment where he could actually see whether there's light beyond the rainbow that we're used to seeing.  So what he did is he shined light, in this case it was sunlight.  We didn't have electricity at the time.  This is always amazing to me that these discoveries were made before people even had electric lights. They were using sunlight. It's incredible. But he passed sunlight through a small opening, and then passed that through a prism, and he saw the familiar rainbow.  But then he did something clever.  He took a thermometer - he knew that when sunlight shined that thermometer got warm.  So he took a thermometer and he put it in different places in the rainbow. Okay, here's red light, green light, blue light, which of these warms up the thermometer more.  And that was a way of measuring how much energy was contained in these different colors of light.  And he found it's not the same.  Some colors of light contain more energy than others.  That was interesting in its own right. Okay, we'll revisit that later a little bit that bluer light has more energy in it.  But the most interesting thing he did is that he went beyond the red part of the spectrum where your eye can see all the colors up to red.  Well, he put the thermometer past the red where it didn't look like there was any light there. And he found that the thermometer still got warm when it was put in that part of the experiment.  So he concluded then that there must be light that’s still there even though his eyes couldn't see it.  There must be some kind of invisible radiation beyond the red part of the spectrum he called infrared light.  Now, what's so cool about this is that it was the first indicator that, wait a minute, maybe there's more to light than what our eyes can detect.

There's a couple different ways we're going to go with this. Most of our video is going to focus on the science applications of this but I want to just pause for a second because I think there's a really interesting sort of living analogy here almost like a parable, and that is that as

Christians, we acknowledge that there is more to this world than we can perceive with our senses.  There is, in a sense, a hidden element to reality, a spiritual element to reality, and we can see that there's a precedent for this. There's an analogy or a parable for this in nature, where there's visible light that can be detected with our eyes but then there are other parts of the spectrum that were previously unknown or undetected.  So I think it's a beautiful analogy to think about, what remains invisible to us today that we can't see yet we detect in other ways; the spiritual reality that we experience.  I think that's a powerful parable that we can take from science and inform our understanding of the spiritual reality that surrounds us.

So here's the thing.  This infrared radiation, that was just the beginning.  There's many other parts of the visible spectrum. But infrared is in particular, a fascinating part of the spectrum.  I’ll show you this picture. This is the third picture now in the gallery.  What this shows on the left hand side is a picture of a man in visible light.  He's wearing glasses, and he's has a trash bag over his hands.  And this is a familiar sight.  This is what anyone looks like we're just walking around, in in the light that our eyes can detect.  But on the right hand side is a picture of the exact same situation in infrared light.  What I want you to notice is that the infrared light passes through different things than visible light.  So, for example, you'll notice you can see through the trash bag in infrared light.  It just passes right through that thin plastic.  Notice also that the man's glasses are completely dark.  So in the infrared, the infrared light cannot pass through the glass of his glasses.  So it's kind of weird.  Things that normally transmit visible light can block the infrared and things that normally block visible light can transmit the infrared.  So light, depending on what part of the spectrum we're talking about, can behave very differently and interact with material in different ways. It's pretty cool.

Okay, but infrared is just a part of the story, just a part of the story.  The full electromagnetic spectrum was sort of discovered and unfolded over the course of many decades between the 1850s and really kind of the early 1900s.  And now what our picture is is that all of these things are really the same stuff.  We call it all the electromagnetic spectrum.  That the light we see with our eyes is really oscillations of electricity and magnetism, which is crazy to think about.  But that the frequency of these oscillations, how rapidly they oscillate back and forth, is what determines whether something can be seen by our eyes, or whether it can't be.  So that frequency can also be thought of in terms of wavelength.  Those are kind of interchangeable ways of thinking about it.  You see that represented in this fourth picture in our gallery, which shows the wavelength across the top, showing very long waves on the left and very, very short wavelengths on the right.  And then you also see that represented with frequency lower down in the chart, which shows very high frequencies on the right and very low frequencies on the left.   there's lots of different ways of talking about the parts of the electromagnetic spectrum.

What I want you to notice, though, I think wavelength is going to be the most useful for us in our class.  So what I want you think about is that these waves, in a sense, have different sizes with their wavelengths.  Visible light, which our eyes see is roughly in the middle. And as the wavelength gets longer and longer, we get to the infrared light that was discovered by Herschel. Now this infrared light comes up in many ordinary applications.  When you think about infrared cameras, you saw that.  Infrared cameras are used with security cameras, sometimes to be able to see at night.  Infrared is also used in remote controls. So when you have that little light bulb on the end of your remote control for a television, that's an infrared light.  It glows in light just a little further red than our eyes can see.  

When you get to longer wavelengths, we get to the microwave part of the spectrum.  Again, it's the same stuff.  It's still oscillating electric and magnetic fields, but now at a longer wavelength and there's different uses and applications.  And so in this microwave region, we have a few applications.  One, obviously, is microwaves.  At a very specific wavelength of light, when the microwaves oscillate back and forth, water molecules can ride along with those oscillations and the water gets hot.  And so what a microwave oven does is it shoots that specific wavelength of light, electromagnetic radiation, onto your food, and it heats up the water inside your food.  So that's why really all microwave can do is boil water, and it just so happens that almost all of our food has water in it, so it heats up our food.  But if you were to put a piece of tinfoil or a piece of metal, there's no water in there and that's when you get the sparking.  You're not supposed to do that with the microwave.  Microwaves have other applications though, too.  Cell phones transmit and receive signals in a part of the microwave spectrum.  A few centimeters is the wavelength.  So microwave has application for communication as well as for actually cooking your food.  

And then we get to the radio wavelengths; we get to longer and longer wavelengths, and those are used primarily for communication.  So you have longer antennas, like on your car, or maybe a radio that you use to listen to the radio.  And that's sending not only voice communication, but also television signals are generally carried on radio wavelengths, things like that.

And on the other side of the spectrum, we have visible light, it's getting on the shorter side, the blue side of the spectrum.  And then when you go past violet you get to ultraviolet.  So now we're getting to higher energies. And when we think of ultraviolet, we probably generally think of harmful UV light from the sun.  That's one way to think about it.  It's higher energy, it's shorter wavelength, it's about the same size as a molecule so it can actually penetrate a cell and cause cancer in cells. That's why it's harmful.  Ultraviolet, also associated with black lights.  You know when you shine a black light and things glow in the dark, that's ultraviolet light that's shining and being absorbed by material and then re-emitted in light our eyes can see.  

We go further towards shorter wavelengths, higher energies, we get into the X rays.  Amazing, amazing thing. Remember, it's the same stuff, just oscillating electric and magnetic fields but now the wavelength is so short, it can pass right through materials pass right through paper, and through walls, and even through our skin, but it reflects off of really hard, dense things like our bones.  So naturally, we know X-rays are used for seeing pictures of our skeletons inside our bodies.

And then we get to gamma rays. Gamma rays really don't have many applications, but they're very high energy waves and they are created in deep space with tremendous explosions. Sometimes nuclear explosions on Earth can cause gamma rays.  They are something we associate with the general term radiation.  When you have a nuclear explosion or nuclear event that is that much energy, you would see the creation of gamma waves.

So there's a sense of the full spectrum of light.  Again, it's amazing.  It's all the same stuff, but different wavelengths, and so many different applications that we can see from them.  In every part of the spectrum, our universe has things that give off that kind of light.  There are things that glow in the radio waves.  There are things that glow in the infrared.  There are things that are gamma ray bursts, these huge explosions that just shower out gamma rays.  We observe bright things in the sky and X-ray light.  So we have telescopes that can observe all of these different parts of the spectrum.  It's really truly amazing.

Now one more thing that is on here that it's really worth taking a look at.  So we're going to see this several times throughout our course.  And that is the bottom part of the of the graph.  It says temperature of objects at which this radiation is most intense the wavelength emitted.  So temperature is also connected to these different parts of the spectrum.  We need to make some sense out of that. 

And there's often confusion that infrared light is somehow heat or heat light.  And that's really a misnomer because here's the thing.  If we go back to the third picture in our gallery, you will notice that this man's face is kind of glowing in that infrared picture.  It seems like he's glowing in the infrared, and he is.  But the truth is, is that everything is glowing all the time.  This is kind of crazy, okay? Everything that has a temperature is glowing. 

So right now, there's probably a light bulb somewhere around you.  That light bulb is glowing in visible light, like you can see with your eyes, because it's glowing so hot.  If you touch it, it probably could burn your hand, maybe; it's really hot.  Where you see the sun, it's glowing brightly, kind of yellow, because it's so hot.  Well, right now, my hands, my face, they're glowing.  They're glowing, but they're glowing in the infrared because if I go back to this fourth picture in the gallery here, the infrared is the things that are a little bit cooler glowing in the infrared, and my body and skin, there may be 70 degrees Fahrenheit, what is that… you know, maybe I'm not good at Celsius.  I'm really sorry if Celsius is your familiar unit of temperature. But what would that be – 70 degrees Fahrenheit is like 20 degrees Celsius? So that's the temperature of my skin.  And at that temperature, it’s glowing in the infrared.  As things get colder, they glow in the microwave, or in the radio wave part of the spectrum. 

On the same token, if things get really hot, they glow at the farther end of the spectrum.  They glow bluer, they glow in ultraviolet, even in the X-ray.  Some of the hottest things in our universe, this material is falling into black holes, it's so hot it actually glows in X-ray light.

So temperature is connected to the light that we give off and it's connected in a very specific way.  And that relationship is called Wiens law.  And you're going to see this a couple times in our course.  So this is another graph.  It's another one of those graphs where it's like, if you want to impress your friends, say, “Hey, I'm learning astronomy, and it's really complicated.”  You're going to show this picture, this is the last picture, our gallery.  

So let me just orient you to the graph a second.  On the bottom scale, we have wavelength, and it's measured in really small units called micrometers.  What that means is that one micrometer, can you see that on there, that's kind of like the infrared part of the spectrum.  The important thing you see there is that the colors of the rainbow are labeled.  So right there, red, orange, yellow, green, and blue, that's where the visible part of the spectrum is on this graph.

On the vertical scale, you see intensity; how much light is shining.  And these curves are showing what we call the spectrum, that is to say how much light is shining at every one of these wavelengths for a given object.  Now, these curves are showing you the amount of light that's being given off at different temperatures.  Now the units of temperature you see here is a capital K, which maybe is not something you've seen before, but that's the scientific unit for temperature that we use in astronomy, it's called Kelvins.  And it's referenced against zero Kelvin being absolute zero, coldest anything can ever get.  So roughly, the units of Kelvin is pretty close to Celsius, you know, but it's Celsius basically, plus 270 degrees.  So when you see 3000 degrees Kelvin, it's basically 3000 degrees Celsius.  So we're talking about hot things here.  Your stovetop when it glows and it starts getting red hot, if you turn it all the way on high, we're at most maybe 1000 degrees Kelvin.  So these are starting hotter than your stovetop.

And I want you to notice here is that when something is hot, like 3000 degrees Kelvin,

it doesn't give off very much light, relatively speaking.  It's glowing. It's glowing primarily in this infrared part of the spectrum, but its tail goes a little bit into this red and orange part.  So think about this.  You and I are cold. We’re like 300 degrees Kelvin. We're glowing in the infrared.

If we were to get hotter and hotter and hotter and start approaching the temperature of a star, we would start glowing in the red part of the spectrum.  Yeah.  Just like your stovetop gets warmer and warmer and it starts glowing red.  And if we got hotter and hotter, two things would happen. One is we get brighter and brighter.  Notice that as you go from 3000 to 4000 Kelvin, you're the intensity goes up quite a lot.  So that's one thing that happens, it's brighter and brighter. And notice 4000 to 5000, it goes up even higher, 5000 to 6000, it jumps up way higher.  So as you get hotter and hotter, you get way brighter, like a lot brighter.  And the second thing that happens is that as you get hotter and hotter, the color that you glow moves towards the blue part of the spectrum.  So you might start glowing red, but then your spectrum is kind of getting more of the yellows and greens and by the time you get really, really hot, you might even start to look like you're glowing blue hot.  You know, like you heard the phrase white hot, that's when you're glowing right here at the top, and you have every color of the rainbow. And every color of the rainbow means it's white light. 

But once you get past being white hot, you get over here to the other side of the spectrum, you're mostly glowing in the blue light. So you like blue hot, that's really hot, and ultraviolet hot. So the point is, as you get hotter and hotter, something gets brighter and brighter and brighter, and it gets bluer and bluer and bluer.  And that's especially true when we're talking about stars.  As we'll see, it helps us learn about stars when we can look at the light that they're giving off.

Alright, that's probably more than you ever wanted to know about light.  But as we'll see, we're going to be taking that information and seeing what we can learn about the universe by carefully studying that light.

 Alright, we'll see you next time.

The Gallery

The Electromagnetic Spectrum - Christian Leaders Institute

Reading: "Invisible Radiation" by Chris Impey

We take in the world through our eyes. Sight is arguably the most powerful and sophisticated sense; the one many of us feel we could not do without. Yet the visible spectrum that we see -- the richness of the rainbow from red to blue -- is just a tiny slice of an enormous array of types of radiation. There is an unseen universe waiting to be explored.

English astronomer William Herschel opened the first chapter in the story of human discovery of invisible radiation. In 1800, he dispersed the Sun's rays with a prism and placed a thermometer beyond the red end of the spectrum. The temperature rose, showing that the thermometer had absorbed invisible solar radiation with a wavelength longer than that of red light. The next year, German chemist Johann Ritter created a spectrum in the same way and placed paper soaked with silver chloride beyond the violet end of the visible rays. The paper darkened, indicating it had absorbed invisible radiation with a wavelength shorter than the shortest wavelength of blue light. Like explorers, these scientists had traveled beyond the rainbow, measuring waves that the eye cannot see.

Around the beginning of the 20th century, another pair of discoveries pried open the spectrum of radiation even further. In his darkened laboratory, Wilhelm Roentgen passed electricity through a tube filled with gas at much lower density than the air. To his surprise, a chemical-coated screen on the other side of the room glowed whenever he passed electricity through the tube. The discovery was accidental, but as any good scientist would have done, Roentgen used logic and further experimentation to try and understand his observation. Light could not be responsible; the room was darkened and the tube was encased in thick cardboard. When his hand passed between the tube and the screen, he was startled to see the bones in his hand, as if the flesh had been stripped away! Newspapers gave prominent coverage to this spectacular discovery. Roentgen had discovered a strange new form of high-energy radiation-- X-rays. He was awarded the first Nobel Prize in physics. In the same year, 1895, young Guglielmo Marconi experimented with long-wavelength radio waves that traveled through space and walls and people unimpeded.

People considered the types of radiation studied by Roentgen and Marconi to be wonderful and mysterious. Today, we take them for granted. X-rays are one of the essential elements of modern medicine, and radio waves are the basis for worldwide communication. How can we use these waves exist that are much shorter and much longer than the waves of visible light to explore the invisible universe? The answer is that in the last fifty years, astronomers have increasingly learned about the universe using electromagnetic waves that are much shorter than and much longer than waves of light.

Reading: "Thermal Radiation" by Chris Impey

Radiation is the principal way that heat and energy travel through the universe. The energy of each and every star, including the Sun, is carried across space in the form of radiation. With our telescopes on Earth, we capture and analyze that radiation. For now, we will focus on the role of radiation in the transfer of heat and energy.

What basic terms and concepts do we need to talk about radiation? Newton was the first to describe the components of radiation emitted by the Sun. He let a narrow beam of sunlight into a dark room and passed it through a prism. The light spread into the same array of colors that you can see in a rainbow. Newton proved that the visible radiation from the Sun is made up of a mixture of light of all colors. The array of colors that Newton saw -- red, orange, yellow, green, blue, indigo, and violet -- is called the visible spectrum. (Many people use the mnemonic "Roy G. Biv" to remember this sequence.) Newton was not the first person to disperse light into a spectrum, but he was the first to systematically deduce light's properties. Some scientists suspected that the colors were not part of white light but were introduced by the prism itself. So Newton passed the visible spectrum through a second prism and showed that it recombined back to white light. White light really is a superposition of colors. But are the colors fundamental? Newton selected one color from the spectrum and tried to disperse it further with a second prism. Blue light remained blue light and red light remained red light. The colors therefore represent a fundamental property of light.

Newton thought of light as a stream of tiny particles. Other scientists noticed that light had many of the properties of waves. As it turns out, it is equally valid to think of light as a wave or as a particle. In 1800, astronomer and composer William Herschel did an interesting experiment. He passed sunlight through a prism as Newton had done before. When he placed a thermometer in each color, the thermometer heated up, since sunlight of any color carries warming energy. Then he placed the thermometer beyond the red end of the spectrum, where no sunlight is visible. Would it heat up, Herschel wondered? Amazingly, it did. Herschel had discovered that there is radiation "beyond the rainbow" that cannot be detected by our eyes. It is called infrared radiation.

The easiest way to think about radiation is to consider its wavelike qualities. When light is spread out into a spectrum, each color corresponds to a different wavelength. Wavelength refers to the length of the wave -- the distance between any two peaks or troughs in the wave. Whenever you see the word "wavelength" in reference to light, you could substitute the word "color" if it helps make the idea clearer. Notice, however, that it is just for convenience that we specify seven colors in the spectrum as listed above. There is actually a smooth and continuous change of color across the spectrum. Similarly, there is a smooth and continuous change of wavelength. Blue light has the shortest wavelength -- about 0.0004 millimeters. Red light has longer wavelengths -- about 0.0007 millimeters. Infrared radiation has wavelengths that are too long for the eye to see -- longer than 0.001 millimeters.

The maximum amount of radiation from the Sun comes in the wavelengths we call yellow -- the wavelengths to which our eye's receptor cells are most sensitive. In fact, this is an example of the way that humans adapt to their environment by evolution. The intensity of radiation declines gradually toward longer and shorter wavelengths. From the combination of wavelengths, we see the Sun as yellowish-white. The spectrum of radiation extends beyond the wavelength range to which our eyes are sensitive. Wavelengths too short for our eyes to detect are called ultraviolet radiation. The Sun emits invisible radiation at both ultraviolet and infrared wavelengths.

Temperature is related to the microscopic motions of atoms and molecules. The larger the kinetic energy of the particles, the higher the temperature of the material. Now we see that particles in motion emit a smooth spectrum of radiation. The larger the kinetic energy of the particles, the shorter the peak wavelength of the radiation. The thermal spectrum depends on temperature in a simple way, given by Wein's law. Since all atoms and molecules are in constant motion, all objects emit thermal radiation. We can also see why the radiation does not depend on composition. If we had a lump of iron and a lump of gold at the same temperature, the iron atoms and the gold atoms have the same kinetic energy. Therefore the iron atoms and the gold atoms emit the same thermal spectrum.

If everything is constantly emitting thermal radiation, why don't we see it? Objects at room temperature emit mainly infrared radiation that we cannot see. Not enough of the radiation comes out in the visible part of the spectrum to be detected by our eyes. We have the technology now to detect and make images with infrared radiation just as we do with visible light. As temperatures increase, the dominant radiation shifts along the spectrum toward bluer or shorter wavelengths. Only when objects reach high temperatures does the dominant radiation move into the visible region of the spectrum. In other words, we can see a radiant glow only from very hot object.

A good example of Wein's law in action comes when you turn on an electric stove. The coil on the stove starts out at room temperature (about 300 K) and is dull gray. This gray color is not emitted by the coil; it is merely the color of the metal as seen by the ambient light in the room. But then the coil heats up, and eventually we begin to see a dull red glow. As the coil gets hotter, the glow becomes brighter and eventually becomes a slightly orange-red. (Molten lava has a similar red glow, and has about the same temperature, about 1100 to 1500 K.) If the coil could get hotter, the radiation would get yellower and finally shift to a mix of colors similar to sunlight, which we perceive as "white" light. Because most objects in daily life are too cool to be "red hot," their thermal radiation is in the infrared, invisible to us.

It is easy to get confused when thinking about color and thermal radiation. We see most ordinary objects by reflected light from the sun or from light bulbs. A blue book is not hotter than a red book; it is just reflecting a different part of the spectrum of a light source. In a room with no light source, a book has no color because there is no light to reflect! We also see the Moon and the planets by reflected sunlight. The only objects that emit their own visible radiation have a temperature of a few thousand Kelvin, like the Sun or the filament of a light bulb. It is important to understand this difference. Now you may be wondering -- what about a fluorescent light bulb or tube, which feels cool to the touch? The gas inside this kind of light source has a very low density. So while the gas atoms have a high kinetic energy that corresponds to a high temperature, the rate of collisions with the enclosing tube is low so there is little heating effect.

Another type of confusion arises from the popular culture. Artists talk about red as a "hot" color and blue as a "cool" color. Musicians use the same terminology -- jazz is cool and associated with the color blue and salsa is hot and associated with the color red. Blood is hot and red, but ice is cool and blue. Unfortunately, this subjective description of color is opposite to the scientific description of color based of thermal emission.