Figure 2-8. Speed of light

TM 9-258

b. Light travels so fast that it was a long time before

to establish the speed of light to an exactness beyond

that required for practical usage.

its speed could be measured. Galileo Galilei (1564-

e. The speed of light is accepted as 186, 330 miles

1642) tried to measure light velocity by means of blinking

per second. This is the figure used in computations

lanterns on widely distant hilltops but, as the speed was

involving the speed of light. The speed of light obtained

too great to be measured by this method, the velocity of

by the important methods of the past 70 years ranges

light was presumed to be infinite.

c. Thirty-four years after Galileo's death, Olaus

from 186, 410 to 186, 726 miles per second.

Roemer (1644-1710) determined the approximate speed

2-3.

Wavelength and Frequency

of light by astronomical means based on observations

a. Wavelength. The wavelength is the distance

which showed the amount of time required for the light of

from the crest or top of one wave to the crest of the next,

the planet Jupiter to cross the orbit of the earth.

or from any point on one wave to the corresponding

d. It was nearly 200 years more before scientists

point, in the same phase on the next wave (fig 2-9). The

were able to measure a speed so great as that of light by

velocity of all forms of electromagnetic waves in vacuum

laboratory methods.

In 1849, Fizeau (1819-1896)

is the same, that is, the speed of light is approximately

measured the velocity of light, using intermittent flashes

186, 000 miles per second. The wavelengths, however,

of light sent out by a source equipped with a swiftly

vary greatly

moving toothed wheel and reflected back to the

observer. Other methods, employing the reflections

from rotating mirrors and prisms, have since been used

2-7