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ELECTROMAGNETIC SPECTRUM

It may seem that astronomers and astrophysicists have a particularly difficult task. They have to study stars, including our Sun, from a great distance. However, we don't have to touch an object to learn about it. Humans are quite experienced at "remote sensing". We receive most of our information about the world from sensors that receive information from a distance; our eyes, ears and nose.The most important tools for astronomers and astrophysicists are those that improve their sight. The invention of the telescope in the early 1600s was one critical step. However, the most important advances have come in our understanding of light itself.

The publication in 1704 of "Opticks" by Sir Isaac Newton was important to our understanding of light . In the process of explaining his theories of light, Newton revealed the results of his experiments with a prism that clearly showed that visible light from the Sun is a mixture of a continuous spectrum of colors.

It is convenient to describe this color spectrum as Red, Orange, Yellow, Green, Blue, Indigo, and Violet (Roy G. Biv). However, it is important to realize that there is a continuous blending of colors from red to orange to yellow and so on. For more on Color.

The important discovery that visible light was made of a continuous spectrum of colors of light led to two other important discoveries in the 1800s. In 1800 the astronomer, Sir William Herschel, performed experiments to understand why different filters on his telescopes seemed to heat up differently. Herschel separated sunlight into its spectrum with a prism. He used three thermometers with their bulbs blackened with soot. He put one bulb in each color of the spectrum and used the other two bulbs as controls. He found that each color of light produced a temperature higher than the controls. He also observed that the temperature increased as he progressed from the blue to the red. When he measured the temperature just beyond the red, he found this region with no visible light produced the highest temperatures.

Further experiments by Herschel showed that he could reflect, refract, absorb and transmit whatever was in this region. Herschel correctly concluded that there was a portion of the light beyond the red (infrared). You can perform The Herschel Experiment at home or in the classroom.

>When Johann Wilhelm Ritter heard of Herschel's work, Ritter reasoned that there might be invisible radiation on the other end of the spectrum - beyond violet. Ritter was an accomplished chemist, and he used his knowledge of chemistry to test his theory. He knew that blue light decomposed silver chloride to silver more efficiently than red light. He reasoned that non-visible light beyond blue might be even more efficient. He was right! When he exposed paper covered with silver chloride to the complete spectrum of sunlight that had passed through a prism, the silver chloride in the region beyond violet (where there was no visible light!) decomposed the fastest. From this experiment Ritter knew that there must be light beyond violet. This radiation came to be known as ultraviolet light.

The new light spectrum included invisible radiation at both ends of the visible spectrum.

The new radiation could be reflected, refracted, absorbed and transmitted just like visible light.

It is useful, at this point, to consider an idea about light presented by Christiaan Huygens in 1690. Huygens explained light as a wave. In the early 1800s Thomas Young supported the wave theory of light with a number of important experiments. (In 1905 Einstein showed that light also has particle properties. We now understand that light has both wave and particle properties.) The theory that light has wave-like properties allows us to consider light in terms of its wavelength. The wavelength can be explained in this simple diagram.

Understanding the crest or trough of a water wave is simple. Understanding the crest and trough of a light wave is not! For a dynamic exploration of wavelengths of light and the nature of light, the Electromagnetic Waves tutorial of Physics 2000© is an excellent resource. Also Imagine the Universe Level I and Level II and Star Light, Star Bright may help.

The wavelength of visible light is measured in Ångstroms (10-8cm) and ranges between 4000Å (violet) and 7000Å (red). Scientists also use nanometers (nm) that are 10-9 meters. In nanometers the range is 400nm to 700nm.

It is sometimes convenient to measure the frequency of waves. The frequency is the number of complete cycles -crests or troughs - that pass a point in a period of time. The number of cycles per second is called a hertz (HZ).

The 1800s were rich with experiments and theoretical advances that added greatly to the work of Herschel, Ritter and Young. Hans Christian Oersted found that an electrical current in a wire caused a magnetic field around the wire. Reacting to Oersted's findings, Michael Faraday was able to generate an electric current with a changing magnetic field. James Clerk Maxwell. Maxwell showed that a few relatively simple mathematical equations could express the behavior of electric and magnetic fields and their interrelated nature; that is, an oscillating electric charge produces an electromagnetic field. Maxwell also calculated that the speed at which an electromagnetic field moved away from its source is approximately that of the speed of light. Furthermore, his equations showed that the electromagnetic phenomena had wave properties. He proposed that the phenomenon of light is therefore an electromagnetic phenomenon. Because charges can oscillate with any frequency, Maxwell concluded that visible light forms only a small part of the entire spectrum of possible electromagnetic radiation.

Maxwell published his work in A Treatise on Electricity and Magnetism in 1873. In 1886 Heinrich Hertz constructed very simple circuits that proved Maxwell's theory. Hertz was able to transmit radio waves (or frequencies) through air to a receiver. Hertz viewed his demonstration as simply verification of Maxwell's math. However, Guglielmo Marconi immediately saw an application - wireless telegraphy. After experimenting for several years he was able to demonstrate trans-Atlantic communication. This was the beginning of the wireless radio and radar.

In the same year that Marconi began experimenting with his wireless transmissions, William Carl Roentgen was discovering another type of electromagnetic emission - the x-ray.

Roentgen experimented with the penetrating power of x-rays. He "took pictures" of his wife's hand, various metal objects (including his gun), wood and glass. The use of x-rays as a medical tool was obvious to Roentgen and others. Roentgen's experimentation with his gun and other metal objects predicted the use of x-rays in airport security and to detect metal fatigue.

The common use of the word 'light' to mean only visible light causes some confusion. Scientists often refer to forms of light as electromagnetic radiation and the entire spectrum as the Electromagnetic Spectrum.

As another way of looking at the different types of electromagnetic emissions, this table provides information on the wavelength and frequency for different "types of light".

These many discoveries and the invention of instruments to allow us to "see" more than the visible wavelengths of light have provided astronomers with important tools to explore our universe from afar. Find out How Astronomers Use the Electromagnetic Spectrum to look at the universe in all of its light.

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