 Greetings and welcome to the Introduction to Astronomy. In this video, we are going to look at a couple of things and that is the idea of light and what the electromagnetic spectrum is. Now, light is how we see everything in astronomy and the only way we can really study most astronomical objects because all we can do to study things like stars and galaxies is to look at the light that comes from them. But the point we want to get across here is that light is not the light that you're necessarily thinking of, which is visible light, but includes an entire range of types of light that go well beyond that. So let's look at some of that here. So what do we mean by light or what is light? Well, light is a form of electromagnetic radiation, sometimes given as EM radiation. It is energy released by moving charged particles and it applies not just to the light that we're used to being able to see, which is this portion over here of the visible spectrum, the colors of the rainbow of red through violet. If we look at what that is, that's expanded here, that is only this very tiny portion of the entire electromagnetic spectrum. There is a lot more to this in terms of objects like radio waves down here, microwaves, infrared, ultraviolet, x-rays, and gamma rays. Up here we have the very high energy types of radiation. So things like x-rays and gamma rays are very high energy radiation and radio waves are down here at the low energy portion. So we have various types, amounts of energy, types and amounts of energy that would then be present. And it all depends on things like the wavelengths or the frequencies of the particles that we'll look at here. But light itself is a very interesting type of object. We sometimes look at it. Is it a wave or is it a particle? And we're going to see that it actually has combinations of both of those. So is light a wave or is light a particle? And what we're going to find out is that it is both. And it is both at the same time. So let's look first at the wave nature of light. A wave pictured here has things like a crest where it reaches its peak, a trough where it reaches the bottom, an amplitude which is how large the disturbance is, and a wavelength which can be measured as the distance from the one crest to the next crest. So we can measure that wavelength. So the different types of light can be characterized by these. And we measure them. We have a wavelength which is just how far apart the waves are. And we also have a frequency which if you imagine standing at one spot and letting the waves pass you, it tells you how many of them pass you every single second. So how fast do the waves come by? Well, you have hundreds or thousands, or when we get to light waves, millions or billions of them passing you every single second. And that would be the frequency. The higher the frequency, the higher the energy. So if you have a very high frequency, that would be things like x-rays and gamma rays. You have a very high energy. If you have a very low frequency, you have a very low energy. Now, in terms of light waves, when you take these two and you take the wavelength of a specific type of light and you multiply it by its frequency, you always get the same number. And that number is 300,000 kilometers per second. And that is what we know is the speed of light in a vacuum. We see the speed of light as a universal speed limit. How fast can something go? Well, there's a limit to how fast it can go, and that is the speed of light. Nothing can go faster than light in a vacuum. Now, the speed of light can change if you're on Earth, if it's traveling through the atmosphere, if it's traveling through glass or any other object, the speed of light can actually change. But in a vacuum, there is a maximum value for the speed of light. Now, what are some ways in which light behaves like a wave? Light waves interfere with each other just as water waves do. If you have multiple water waves together, they can add together and make much larger waves or subtract from each other and make much smaller waves. Light waves can do the same thing. We also see things like reflection, refraction, and diffraction of light waves that would only occur if light has a wave nature. And finally, the Doppler effect, how we measure velocities in astronomy, would only occur if light had a wave nature. So this seems to show us that light is a wave, but light also has particle properties. And we want to look a little bit about those. When we look at particle properties, in this case, particle properties are that we have the particle nature of light, and we see light as a bunch of photons. Photons are packets of energy, and they can be particles much like an electron or a proton. The difference is that they are essentially massless and that that's the only way they can then travel at the speed of light. We can determine the energy of a photon by calculating its frequency. If we know what its frequency or wavelength is, we multiply it by a constant here, and that gives us the energy of that photon. So a photon with a very high frequency has a very high energy and a very low frequency would have a very low energy. Now, one of the big ways that photons are seen to be particles is through the photoelectric effect. Now, this was seen by Einstein about almost 100 years ago or so that when you could shine light, shine photons on a piece of metal, that if you had a sufficient amount of energy, they would be able to release electrons from the metal. So if you had the right amount of energy, the electrons would be released from the metal and be able to, for example, then travel to another piece of metal and complete a circuit. So it was an example of the photoelectric effect. If you had insufficient energetic photons, for example, if this needed, say, blue light to be able to eject the photons, you needed the energy of a blue light photon to be able to inject this. Then you had that you could shine all the red light you wanted onto it as bright as you could and you would not be able to complete the circuit because those red photons did not have sufficient energy. But as soon as you got down to the blue photons, as soon as you got down to that right energy, then you were able to release those and have the electrons then be released. Another way is through gravitational deflection. Einstein's general relativity tells us that light will be bent as it passes near a massive source, and that is another way in which light behaves like a particle, as waves would not have this same type of effect. So let's look at the entire electromagnetic spectrum here. Again, it looks at all the different wavelengths, everything from gamma rays over here through radio waves over here, and those types of radiation, as we've already discussed, depend on the amount of energy. So very high energy, high energy, low energy over here. And it depends on the wavelength. So these have a short wavelength and these have a long wavelength. Now, certain types are able to get down to the ground. For most of history, of astronomical history, up until about 100 years ago, we used the optical window. That was how everything was observed, and that was through that very tiny portion of the electromagnetic spectrum that we know is visible light. In the 1930s, we added in the radio window. It was always there, but that's when we had developed the technology that would allow us to be able to detect radio waves from space and get a completely new picture of the universe. Now, to observe some of the others, like x-rays and gamma rays, we had to get above the atmosphere, so it wasn't until rockets in the 1960s that allowed us to get up and finally launch satellites that would allow us to observe things like gamma rays and x-rays and even study things like ultraviolet and infrared in far more detail. So the entire electromagnetic spectrum is now giving us a more complete picture of what the universe looks like. Now, let's take a little aside here and look at temperatures, because temperatures are important for these objects. So what do we mean by temperature? Well, the temperature is the measure of the average kinetic energy of the particles. So if you have a glass with a liquid in it, how fast those particles are moving will tell you the temperature. The faster you get them moving, the higher the temperature, the slower you have them moving, the lower the temperature. Now, that kind of begs the question, what would happen if you could stop those particles from moving altogether, if you could continue to decrease the temperature? In that case, we would reach what we call absolute zero, as cold as things can possibly get. Now, pictured here are a couple of the different temperature scales. In the U.S., we use the Fahrenheit scale. In most of the rest of the world, we use the Celsius scale. However, astronomers will use the Kelvin scale for the vast majority of their work. And the Kelvin scale is set up based on this absolute zero. Absolute zero is as cold as you can possibly get because once things stop moving, you can't make them any colder. So this would be zero Kelvins or absolute zero. And space would be three Kelvins. So the temperature of outer space out where nothing exists, out in the depths of the universe, would be about three Kelvins. That's as cold as things get. So particles are not moving much at all. Now, as for a comparison, at Celsius, we would read that as negative 273 Kelvin. So no matter how cold it gets, we never are getting anywhere near absolute zero here on Earth. And the Celsius scale, zero degrees is where water freezes, and that would be 273 on the Kelvin scale and 32 degrees Fahrenheit on the Fahrenheit scale. Now, we can convert between these two types. Conversion between Celsius and Fahrenheit is a little more complex, and I'm not going to go into that here. Between Kelvin and Celsius, it's very easy. It's just an offset of 273 degrees. So if you have a temperature in Kelvin and you want to convert it to Celsius, you subtract 273 degrees. If you have a temperature in Celsius and you want to convert it to Kelvin, you just add 273 degrees. So it's simply an offset. Nothing more complicated than that to be able to convert between Celsius and Kelvins. And honestly, when we get up to many of the temperatures that we talk about in astronomy, when you're talking about thousands, tens of thousands, and hundreds of thousands of degrees, it really doesn't matter whether you're talking about Kelvin or Celsius, because at the center of the sun, the temperature is about 15 million degrees. Does it really matter, though those 273 degrees really matter at that point? And in effect, they do not. So our last topic for this section we want to look at is the radiation laws. Now, radiation laws, and there are two of them that we'll look at, are based on the idea of black-body radiation. And black-body radiation is an ideal radiator. And that means that it absorbs all of the light incident upon it. That's how it gets its name. A black object would be a much better black body than a white object, because the black body is not reflecting very little light. So an ideal radiator, it would absorb everything on it, and it would give off light based only upon its temperature. So stars are an example of this. They are a pretty good example of a black-body radiator. And the type and amount of radiation that they emit depends only on their surface temperatures. Now, let's look at these two laws. The first one, the Stefan-Boltzmann law, tells us that the higher the temperature, the higher the intensity at any wavelength. So when we look at these stars in the graph here, we have stars ranging from 3,500 Kelvin to 5,500 Kelvin. If you look at the curve for 5,500 Kelvin, it is always above the curve for 3,500 Kelvin. So while it emits more light, it emits more energy at all wavelengths, overall, no matter what wavelength you pick, even out here someplace, if we look at this wavelength right here, then we're still getting more light from this star than we are from this star. So what it tells us is that the higher the temperature, the higher the intensity, the bigger the curve is going to be, and that it does not matter at what wavelength you look. So even though this cooler star emits more of its light in the red, it still means that the hotter star is going to be emitting even more than that. So it is going to be emitting even more in the red than this cooler star. It's just a matter of where the peak occurs. It emits most of its light in the green. This one emits most of its light in the red and infrared. But in terms of the amount, the hotter star, hotter object is always emitting more energy at every wavelength. Now the second radiation law is Wien's law. Wien's law tells us that the higher the temperature, the shorter the peak wavelength. So that's the shift that we're seeing here. Let's clear this up a second. So the shift that we're seeing here is that if you notice the peak is going from here to here to here to here, and it is shifting towards the right-hand side. So the higher the temperature, the shorter the wavelength, and the cooler the object, the different color it's going to look. So this very cool object is going to look red in color because most of its light is being emitted in the red and the infrared. A very hot object would look bluer in color because most of its light would be emitted in the blue. So the peak wavelength does change with temperature as well. Extremely hot objects would give off ultraviolet. Very hot stars emit most of their energy in the ultraviolet part of the spectrum. Very cool stars emit most of their energy in the infrared part of the spectrum. So in those, we do not even see most of the energy that they're emitting when we look at visible light. That's one of the reasons it's very important for astronomers to study all various, all the wavelengths and all the types of electromagnetic radiation. So finishing up with our summary here, what have we gone over? Well, the electromagnetic spectrum itself is more than just visible light. So it is not just the visible light, but it includes all of those other parts that we see, x-rays, gamma rays, and radio waves as some examples. We talked that light has a dual nature. It behaves as a wave and a particle simultaneously. Various observations that we make of light can only be explained if it's a wave, and others can only be explained if it's a particle, and that gives light this dual nature. And finally, we looked at two radiation laws that tell us how the spectrum will vary with the temperature. Hotter objects giving off more energy and more high-energy radiation. So that concludes this lecture on light and the electromagnetic spectrum. We'll be back again next time for another topic in astronomy. So until then, have a great day, everyone, and I will see you in class.