194 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1930 



It is a metric unit, and represents iooo^ooo tt of a millimeter, the 

 millimeter being equivalent to about -^ of an inch. Now in terms of 

 this unit visible light ranges from about 3,600 Angstrom units (A), 

 to about 7,700 A — although it must be remembered that the human 

 organs of vision vary in their range, and that many persons can not 

 see the raj'^s of wave lengths as short as 3,600 A, nor those of wave 

 lengths as long as 7,700 A. 



loooA loooA ioqol 4oooA 5000A 6opoA 7000A 8000A 



B 



w 



-ULTRA 



INVIS 



VIOUT 



IBLC- 



VIOLET Dl 



•-> GAMMA RAY 



^700A 



VISI5LE RAVS- 



>XllOW RqD 

 DC GRHNJ ORANOE 



l\<rRA RED 



7700 A 



imm^m^^; 



^^S: 



\\\^:mx\\\\' 



K 



CV1 ODT 



SS 



k^^^^m^^^^^ 



^ 



SUN RAblATlON A$ IT REACHES THE 

 -V.V.^ -« VISIBLt 





r-v 



ISIBLE 

 RAOIATIOIM 



RAVS 



COMMON 



GLASS 1 



RADIATION A» IT REACHES THE 



MIBCURY VA»OR 



■ DORNO REGION (3i9O0-5(50A) 



FlGDBE 1 



WAVE LENGTHS OF ULTRA-A^OLET 



Wave lengths shorter than 3,600 A, and down to about 1,000 A, 

 constitute the ultra-violet radiation. The shorter the waves, the 

 higher the frequency, that is, the larger is the number which pass a 

 given point in a unit of time. All solar radiation travels at the same 

 inconceivable speed of about 186,500 miles a second, thus being capa- 

 ble of encircling the earth more than seven times in that short period. 

 This applies to ultra-violet as well as to visible light, just as it does 

 also to the very long waves which carry our radio programs. This 

 means that the rays from ultra-violet lamps reach us so speedily — 

 if they are not absorbed by an intervening medium — that we have no 

 time to side-step them, even though we may stand at some distance 

 from the lamp. We can protect ourselves only by the use of an in- 

 tercepting medium. 



The radio waves, many city blocks in length, the infra-red, visi- 

 ble light, ultra-violet, and X rays, all seem to be fundamentally the 

 same, except in the matter of wave lengths and conversely in the 

 rapidity with which the waves follow one upon the other; but by 



