## APPENDIX: The Nature of Light

We are used to thinking of light as that which allows us to see. But this is much too restrictive a definition. What we normally call light is better described as visible light. Light, in general, includes all parts of the electromagnetic spectrum &mdash from radio and microwaves, to infrared, visible, and ultraviolet light, to x-rays and gamma-rays. These are all different types of the same basic phenomenon. Figure 14 is a representation of the electromagnetic spectrum, with different types of light labeled.

 Figure 14 &mdash A representation of the electromagnetic spectrum. Radio waves have the longest wavelength and lowest frequency, while gamma-rays have the shortest wavelength and highest frequency. The pictures at the bottom show real-world objects that are about the same size as the wavelength of light in each different part of the spectrum. Image from http://www.andor.com/library/light/.

The electromagnetic spectrum is a word that scientists use to refer to the wide range of electromagnetic waves. Electromagnetic waves are simply waves of energy. This energy is carried by electric and magnetic fields. Figure 15 is a representation of one of these waves. All waves can be characterized by their wavelength and frequency. The wavelength is the distance between two peaks on the wave. Radio waves have a very long wavelength while gamma-rays have a very short wavelength. Frequency is the number of peaks that pass by an observer every second. Since the peaks of radio waves are separated by a large distance, few of them pass by an observer in a second, i.e. they have a low frequency. Gamma-rays, on the other hand, have a short wavelength, and thus a high frequency. Color is really nothing more than the response of our eyes to different frequencies of visible light. Red light has the lowest frequency and blue light the highest. In radio astronomy, when we talk about the observing frequency, we are referring to the frequency of the light that we are observing.

 Figure 15 &mdash A representation of an electromagnetic wave. The electric field and magnetic field waves are always perpendicular to each other, and to the direction in which they travel. The wavelength is the distance between any two peaks. Image from http://www.pas.rochester.edu.

It is a common misconception that radio waves are somehow related to sound. The only similarity is that sound also travels in waves. Other than that, there is no more similarity between radio waves and sound than there is between visible light and sound. However, it is possible to encode information in radio waves (or any form of light), and that information can then be changed into sound. This is how a car radio or cell phone operate.

The fact that light carries electromagnetic energy is a clue to how light is produced. The simple description, is that light is produced any time there is source of accelerating electrical charge. This is why electronic equipment tends to give off radio waves. Whenever the current flowing through the system changes speed or direction, it will give off light. And most electronic equipment gives off light in the form of radio waves. For this reason, electronic equipment around a radio telescope must be well shielded or removed completely. Otherwise, it will be a source of RFI.

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