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 118 different elements currently on the periodic table. Several elements have only been found in laboratories and nuclear accelerators. So, you may wonder how many elements can be found naturally. The usual textbook answer is 91. Scientists used to believe that, with the exception of the element technetium, all the elements up to element 92 could be found in nature. However, it turns out there are other elements that occur in trace amounts naturally. 1  This brings the number of naturally occurring elements to 98. Technetium is one of the newer elements added to the list. 2  All six of these elements have been found in very small amounts in samples of uranium-rich pitchblende.