Radiation is the general term for light and other forms of electromagnetic radiation that carry energy through the universe, or from one place to another. It turns out that light is just one small sliver of a large spectrum of radiations that have the same fundamental properties that vary in wavelength, frequency, and energy. This was demonstrated with beautiful experiments about 200 years ago. Newton had taken light and shown that it spreads into a spectrum of constituent colors ranging from the bluest blue to the reddest red with a prism. Around the same time, people wondered if there might be radiation beyond the edge of the spectrum. In the year 1800, both ends of the spectrum were investigated. It was shown that there is temperature by placing a thermometer beyond the reddest red that the eye can see. So there is invisible radiation beyond the reddest red, therefore beyond the edge of the visible spectrum. When certain chemically treated paper is placed beyond the bluest blue you can see with the visible spectrum, there is also radiation there. The paper is fogged. Energy is carried by invisible radiation bluer than blue. These demonstrations that energy exists beyond the visible spectrum of light were continued and extended in the 20th century, with technology that allowed electromagnetic waves of very short wavelengths and very long wavelengths to be created. The entire electromagnetic spectrum eventually came under study. We characterize the electromagnetic spectrum with different wavelength regimes, going from the shortest wavelength, or highest energies and highest frequencies to the lowest. We go from gamma rays, to X-rays, to ultraviolet radiation, to the visible spectrum, to infrared radiation, to microwaves, and then radio waves. These characterizations or boundaries are arbitrary and artificial. It's a smooth and continuous variation in wavelength or energy. All of these energies can be created with suitable technology. Remember, in this entire spectrum of radiation, the range of energy, frequency, or wavelength from the highest energy gamma ray, to the lowest energy radio wave is more than a factor of a trillion. Yet optical light represents only a factor of two, from the bluest blue to the reddest red that the eye can see. Naturally, our eyes, our more sensitive optical detectors were tuned to the radiation put out by our star. So our sensory apparatus is tuned to electromagnetic radiation produced by the life-giving body out in space. There are life forms for example on Earth, it sends electric or magnetic fields, or infrared radiation, even occasionally radio waves. Electromagnetic radiation is used by living creatures accordingly to their environment. It may not seem obvious in your everyday life but there's a fundamental unity between the X-rays taken in your dentist's office, the radio you listened to, the infrared radiation that might reach you, that heats you up, and the light that you see with your eyes. These are all fundamentally the same type of entity, electromagnetic radiation. Another way to think of radiation in the universe is according to temperature, which is proportional to energy. It's another way to think of energy. So we've seen that the visible spectrum of light that you can see with your eyes is not the limit to these possible radiations. There are radiations that are higher energy or hotter than your eye can see, and lower energy or cooler than your eye can see. Every object in the universe has a temperature. That temperature gives a sense of where it's thermal radiation comes out. The thermal radiation as we've seen, is based on the microscopic random motions of the atoms or molecules in any object. In other words, thermal radiation resulting from temperature says nothing about what an object is made of. A banana or an apple or a human being at 300 degrees Kelvin emit the same thermal radiation. All the radiation tells you is the temperature of the object. According to temperature, that says where the radiation will appear relative to what we can see with our eye. So let's look at some objects and see how their radiation appears. The Sun appears as a yellow star in the sky. So we might imagine that its radiation is a balance between blue and red light. Indeed the temperature of the Sun at it's surface is about 5,800 Kelvin. Almost any object in space at that temperature will appear yellow to our eyes. The peak of its thermal radiation is sitting in the peak of the sensitivity of the human eye. But objects cooler than that are still emitting radiation. That radiation however will be shifted to longer wavelengths into the infrared. An objects much hotter than the Sun may emit some visible light but the bulk of their radiation will be shifted to shorter wavelengths or higher frequencies ultraviolet radiation. To get a small everyday sense of this trend just imagine what happens as you turn on the electric burner on your stove. When you first turn it on with very little heat going to it, there's no glow. It's sitting there at room temperature. If it has radiation at all, that radiation is at long invisible infrared wavelengths. As the element heats up, it starts to glow dull red, which means its temperature has been raised so that its thermal emission is shifting into the visible wavelength range. If it gets hot enough, it will start to glow yellow and perhaps even white hot. This is a continuous change of its temperature and the peak wavelength of its thermal emission. Taking away the source of energy and watching it cool down goes through the same change in reverse. Objects that are at temperatures of some thousands of Kelvin emit visible radiation like the surface of the Sun, objects that are at temperatures of a few 100 Kelvin, such as the Earth and humans on the earth are emitting far-infrared radiation. If you had an infrared camera, you may have seen some of these at science museums, you can actually see the infrared radiation that all objects at room temperature emit. This behavior of radiation was codified into two very important physical principles that underlie a lot of astronomy and astrophysics too.The first is Wien's law, which states that the temperature of an object scales with the wavelength of its emission. It's an inverse relationship. If you double the temperature of an object, the wavelength of its peak thermal emission halves. That's just the same trend that we've been talking about. The second part of this is called the Stefan-Boltzmann law, which says that the amount or intensity of radiation from an object varies very strongly with its temperature, actually as the fourth power. If you have a particular object and you vary its temperature by a factor of two upward, the amount of radiation coming from it will go up by two to the fourth power or a factor of 16. These two principles underlie stars and the relative amounts of radiation and the peak wavelength of the radiation from different types of stars as we'll see later. Visible light is a small sliver in a huge range of radiation called the electromagnetic spectrum, ranging from extremely high energy, short wavelength, and high frequency such as gamma rays and X-rays, down to very low frequency, very long wavelength, and very low energy such as radio waves and microwaves. All of these are fundamentally the same phenomena. Electric and magnetic waves propagating through space at the speed of light, 300,000 kilometers per second. As the temperature of an object varies, the thermal radiation released by the random motions of its atoms and molecules shifts through the electromagnetic spectrum. Objects at thousands of Kelvin are emitting visible light like the Sun, objects at hundreds of Kelvin like the planets or humans on the planet are emitting far infrared radiation.
