Each case of communicable disease is the result of an orderly progression in. a series of events. This series of events may be described as a three-link chain, each. link representing a factor essential to the transmission of disease. These links (figure 1-1) are: (1) The source of the disease (reservoir) (2) The means by which the disease may be transmitted (mode of.
Radio Waves. Radio waves are a type of electromagnetic (EM) radiation with wavelengths in the electromagnetic spectrum longer than infrared light . They have have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves, radio waves travel at the speed of light. Naturally occurring radio waves are made by lightning or by astronomical objects.
There are three basic types of seismic waves â P-waves4, S-waves5 and surface waves. P-waves and S-waves are sometimes collectively called body waves. P-waves, also known as primary waves or pressure waves, travel at the greatest velocity6 through the Earth. When they travel through air, they take the form of sound waves â they travel at the speed of sound (330 ms-1) through air but may travel at 5000 ms-1 in granite7. Because of their speed, they are the first waves to be recorded by a seismograph8 during an earthquake.