“The human ear can detect only the sound which is produced through from 32,000 to 38,000 vibrations per second.
As the rate of vibrations per second increases above that which we call sound, they begin to manifest themselves in the form of heat. Heat begins with about 1,500,000 vibrations per second.
Still higher up the scale, vibrations begin to register in the form of light. 3,000,000 vibrations per second create violet light. Above this number vibration sheds ultra-violet rays (which are invisible to the naked eye) and other invisible radiations.” -Napoleon Hill