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4.15 Thermal Radiation

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.

Schematic animation of a continuous beam of light being dispersed by a prism. The white beam represents many wavelengths of visible light, of which 7 are shown, as they travel through a?vacuum?with equal?speeds c. The prism causes the light to slow down, which bends its path by the process of?refraction. This effect occurs more strongly in the shorter wavelengths (violet end) than in the longer wavelengths (red end), thereby?dispersing?the constituents. As exiting the prism, each component returns to the same original speed and is refracted again. Click here for original source URL.

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.

Wilhelm Herschel, German-British astronomer. Click here for original source URL.

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.

A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths. Click here for original source URL.

Wavelength of a?sine wave, ?, can be measured between any two points with the same?phase, such as between crests, or troughs, or corresponding?zero crossings?as shown. Click here for original source URL.

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 solar spectrum peaks at yellow wavelengths. The sun also emits radiation at wavelengths not visible to the human eye. Click here for original source URL.

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.

A thermographic (infrared) image of a dog, in false color, indicating heat emitted in the infrared part of the spectrum. Click here for original source URL.

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 Wien'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.