Wave Motion and Interference
17
ment which separates the glasses, it will be found to be, say, two and seven-tenths microns.1 Counting the number of dark bands in red light, we find there are eight; and hence we conclude that at the thickest part of the air film the retardation is eight waves, and hence the distance separating the glasses — that is, the thickness of the filament— is four waves, which gives about sixty-eight hundredths of
-V ■B
FIG. 17
a micron for the wave length of red light. If blue light is used, there will be twelve dark bands, whence the wave length of blue light is forty-five hundredths of a micron.
The following table gives the approximate wave lengths of the principal colors:
Red - 0.08 microns
Orange - - - .63 “
Yellow - - - .58 “
Green - .53
Blue - .48
Violet - - - .43
Fig. 17 gives a diagram of the wave lengths of the different colors, magnified about twenty thousand times.
SUMMARY
Waves give information concerning direction, distance, magnitude, and character of the source. Light does the same; hence the presumption in favor of the hypothesis that light consists of waves.
i A micron is a thousandth of a millimeter, or about a twenty-five thousandth of an inch.
18
Light Waves and Their Uses
Wave trains may destroy each other by “interference.” Light added to light may produce darkness.
The reason why interference is not more frequently apparent in the case of light is that light waves are exceedingly minute.
By the measurement of interference fringes it is possible to measure the length of light waves, and the results of such measurements show that the wave lengths are different for different colors.