Radiation is a specific type of heat transfer. Let’s review some things about heat transfer. In physics, we define heat as a form of energy that is transferred among different substances. Generally, there are three different ways that heat can be transferred (Figure 1). Convection is the transfer of heat through gas or liquids. One exam-ple would be the hot water vapor that rises when you boil water. Conduction is the transfer of heat between bodies of matter. Heat flows from a body of matter at a higher temperature to a body of matter at a lower tem-perature. Conduction is the main mechanism that heats up a pot sitting on a hot burner, where heat transfers from the burner to the pot. A third form of heat transfer is radiation, which describes the movement of energetic particles or waves through space or through substances. Radiation is commonly differentiated into two separate categories: ionizing and non-ionizing radiation.
Ionizing radiation carries enough energy to remove electrons from an atom or molecule, thus producing charged particles. Remember that each element on the periodic table has a certain number of protons, which are positively charged. Each element has the same amount of electrons floating on the outer cloud—these are negatively charged to make the net charge of the ele-ment approximately neutral (Figure 2). Ions form when the element either gains or loses electrons, thus causing the particle to be charged. Examples of ionizing radia-tion include far UV, X-Ray and Gamma Rays (Figure 3).
Non-ionizing radiation is the term given to radiation in one part of the electromagnetic spectrum that does not carry enough energy to ionize particles. Radio waves, microwaves, infrared, visible light, and near ultraviolet (UV) are all examples of electromagnetic radiation. The various types of radiation are differentiated by their wavelength (λ) and frequency (f), which have an inverse relationship (Figure 3).
Wavelength is the distance between the same phases of each wave and frequency is the number of times the wave “peaks” (a complete oscillation) in a given amount of time (e.g., per second) (Figure 4). So, 100 kHz, means there are 100 x 103 oscillations every second (kilo = 1000 = 103).
Waves with shorter wavelength tend to travel faster, or, have high frequency. These waves contain higher energy than their counter parts, which have longer wavelengths and therefore lower frequency. Since radio waves have the longest wavelength and travel the slowest, they also contain the least amount of energy. On the other hand, UV, X-ray, and Gamma rays contain very high energy—atoms can get transferred to molecules in living cells, causing a lot of damage. By the way, the product of the wavelength (in meters) x the frequency (Hz) is actually equal to the speed of light (~3 x 108 meters/sec).