Figure 2-9. Wavelength

TM 9-258

b. Frequency. The frequency is the number of

Special Units of Measurement Employed

in Measuring Wavelengths.

Wavelengths of

waves occurring per second. It is determined by dividing

the speed of light by the wavelength. It is the number of

electromagnetic waves range from many miles long to

waves passing a given point in 1 second (fig 2-10).

those measured in trillionths of an inch.

The

measurements cover such a wide range and the shorter

wavelengths are so minute that special units of

Velocity

measurements have been provided to avoid long

Frequency =

decimal fractions of millimeters or inches.

Wavelength

(1) The first of the special units of

measurement is the micron, which is abbreviated to ,

the Greek letter "mu". It represents one one-millionth of

a meter or one one-thousandth of a millimeter. The next

is variously called the milli-micron and micro-millimeter.

It is abbreviated to m and represents one one-

thousandth of a micron. Finally, the micro-micron is

abbreviated to go. It represents one one-millionth of a

micron.

(2) Another important measurement is the

Angstrom unit (AU) which is one-tenth of a milli-micron

or one ten-millionth of a millimeter. Even Angstrom units

are inconveniently long in measuring the shortest

electromagnetic waves so the X-ray unit (XU) is used for

this purpose. It is one one-thousandth of an Angstrom

unit.

2-4.

Light Rays and Other Symbols Used in the

Diagrams.

a. Every point on a luminous body or an illuminated

object sends out a constant succession of wave fronts in

all directions. The action of light in passing through a

Figure 2-10. Frequency

lens, for example, might be as shown in A, figure 2-11.

But for simplicity, light will be indicated in this manual by

one, two, or more representative "light rays." These "light

rays" are shown as lines with arrowheads indicating the

direction of travel and are shown as in B, figure 2-11.

Wherever possible light will be indicated as coming from

the left side of an illustration.

2-8