LIGHT FILTERS 



293 



length of one complete wave, measured from the corresponding portions of two succes- 

 sive waves; frequency is the number of complete waves passing a given point in a given 

 time, the second being usually taken as the standard time interval. If c is the velocity 

 at which electromagnetic waves travel, the relation between the wavelength X and the 

 frequency / is given by 



c=A (1) 



from which 



(2) 





and 



x = 5 



(3) 



The velocity of light in vacuum has been measured by Michelson to be 2.99796 X 

 10^° cm. per sec. or 186,284 miles per sec. with a probable error of about 1 part in 

 95,000. For ordinary precision c can be taken to be 3 X 10^° cm. per sec. 



Angstrom Units 

 o o o o o o o 



C> C> O lO) o <^ <^ 



g- A °B C S-D E S-F G °-H °' °' 



Infra 



red 



Red-Orange-Yellow-Green-Blue-Violet 



■5° 



3^- 



.' 3 0.2,-' l-_i 



U — Limits of human vision -v 



k Panchromatic film > 



■<- Ortho film — > 



-* Ordinary film — ^ 



Special films -* 



Range of photo cell sensitivity -^ 



Fig. 2. — Wavelength chart, illustrating the range of sensitivity of the human eye, and 

 various types of film emulsions. 



Figure 2 shows the values of wavelength and frequency for various types of radiant 

 energy. For visual and photographic effects the most important range of radiations 

 are those lying between 400 and 700 myu* in wavelength, since this range forms the 

 visible spectrum. Wavelengths of from 200 to as much as 1200 m/x can be recorded 

 on photographic plates commercially available, although the range of useful sensitivity 

 of most photographic materials is from about 380 to 540 m/x for ordinary noncolor- 

 sensitive or orthonon materials, 380 to 600 m^ for orthochromatic materials, and 380 

 to 700 m^t for panchromatic materials. Radiations between 275 and 315 m^ produce 

 sunburn; the X-ray region extends from 30 to 0.01 m/x, and radio waves may have a 

 wavelength of from less than 1 cm. to 25,000 m. 



Since any device producing radiant energy nearly always produces radiations of 

 several wavelengths or a band of wavelengths rather than at a single isolated wave- 



* The wavelength of electromagnetic radiations is usually measured in meters (m) or submultiples 

 of the meter. For radio work the meter is the standard wavelength unit although with recent advances 

 in very short waves the centimeter (0.01 m.) is sometimes employed. For shorter waves, still smaller 

 units are employed. 



The micron, which is one-millionth part of a meter (10~s m.), is sometimes used for the unit of 

 wavelength, although this is still too large a unit to give convenient numerical figures when dealing 

 with wavelengths of visible light. For visible light, a common wavelength unit is the millimicron, 

 which is one-thousandth of a micron, or one one-thousandth-millionth part of a meter (10"' m.). 

 Another unit often used in the visible spectrum is the angstrom unit (lO'i" m.) which is equal to 0.1 m^- 

 The symbol for a micron is ^; for a millimicron, m/t; for a centimeter, cm; for a meter, m; and for 

 an angstrom unit A. Therefore, l/i = 1000 m/j = 10,000 A. = IQ-* cm. = lO-s m. 



