 of red light from about 620 to 760 mm, or from about 0.000016 to 0.000018 in. for violet, and from 0.000025 to 0.000030 in. for red. In many physics books you will note wavelengths expressed in different terms, with, more than likely, the term used based on the age of the book. At one time it was popular to express wavelengths in terms of cm and in., today it is pop- ular to describe the wavelength of light in terms of nm. Appendix A Another term that comes into play when discussing light is the Angstrom. In the case of light, the wavelength is SO short that a specific distance, called the Angstrom (Ä), has been defined as follows: 1A=10-10 m or 10-8 cm Visible light has a characteristic wavelength in the range of approxi- mately 3900 to 7700 A. Electromagnetic energy outside this range is no longer visible to the human eye. Another term sometimes used to express wavelength is the micron (10-6 m), which is a thousand times greater than the nm. The term mu represents an abbreviation for micrometer and is commonly used by astronomers. In the event you are curious as to why the Greek symbol mu was not used, according to legend it was not available in the typeset used initially by astronomers, resulting in mu being used to form the abbreviation micrometer. The following table indicates the representative wavelength of visible colors in terms of mu that correspond to our friend, Mr. Roy G. Biv, mentioned earlier in this book. Color Representative wavelength (microns) Violet Indigo 0.44-0.45 Green Yellow Orange The following table indicates the relationship between wavelength in nanometers, Angstroms, and microns. This relationship is for visible light through the infrared range. Appendix A Wavelength Nanometers Microns Ängstroms 4,000 4,500 5,000 5,500 6,000 6,500 7,000 7,500 8,000 8,500 9,000 9,500 1,000 10,000 1,050 10,500 1,100 11,000 1,150 11,500 1,200 12,000 1,250 12,500 1,300 13,000 1,350 13,500 1,400 14,000 1,450 14,500 1,500 15,000 1,550 15,500 1,600 16,000 1,650 16,500 Appendix A Wavelength Nanometers Micron