Chapter 4: Matter and Energy in the Universe

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 in 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. While exiting the prism, each component returns to the same original speed and is refracted again.

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. The 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.

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 solar spectrum peaks at yellow wavelengths. The sun also emits radiation at wavelengths not visible to the human eye.

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.

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 the 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.