How are wavelength and frequency of electromagnetic radiation related

Electromagnetic Radiation.

Electromagnetic radiation is an electric and magnetic disturbance traveling through space at the speed of light (2.998 × 108 m/s). It contains neither mass nor charge but travels in packets of radiant energy called photons, or quanta. Examples of EM radiation include radio waves and microwaves, as well as infrared, ultraviolet, gamma, and x-rays. Some sources of EM radiation include sources in the cosmos (e.g., the sun and stars), radioactive elements, and manufactured devices. EM exhibits a dual wave and particle nature.

Electromagnetic radiation travels in a waveform at a constant speed. The wave characteristics of EM radiation are found in the relationship of velocity to wavelength (the straight line distance of a single cycle) and frequency (cycles per second, or hertz, Hz), expressed in the formula

c=λv

where c = velocity, λ = wavelength, and v = frequency.

Because the velocity is constant, any increase in frequency results in a subsequent decrease in wavelength. Therefore, wavelength and frequency are inversely proportional. All forms of EM radiation are grouped according to their wavelengths into an electromagnetic spectrum, seen in Figure 1-3.

The particle-like nature of EM radiation manifests in the interaction of ionizing photons with matter. The amount of energy (E) found in a photon is equal to its frequency (ν) times Planck's constant (h):

E=νh

Photon energy is directly proportional to photon frequency. Photon energy is measured in eV or keV (kilo-electron volts). The energy range for diagnostic x-rays is 40 to 150 keV. Gamma rays, x-rays, and some ultraviolet rays possess sufficient energy (>10 keV) to cause ionization.

The energy of EM radiation determines its usefulness for diagnostic imaging. Because of their extremely short wavelengths, gamma rays and x-rays are capable of penetrating large body parts. Gamma rays are used in radionuclide imaging. X-rays are used for plain film and computed tomography (CT) imaging. Visible light is applied to observe and interpret images. Magnetic resonance imaging (MRI) uses radiofrequency EM radiation as a transmission medium (see Fig. 1-3).

Electromagnetic Radiation

EM radiation is a form of energy propagation where photons with both particle and wavelike properties travel at the speed of light.1 EM waves carry energy and transfer their energy upon interaction with matter. The energy associated with EM radiation is proportional to frequency and inversely proportional to wavelength. Thus, EM waves with shorter wavelengths have more energy. Examples of EM radiation (from lowest to highest energy) include radio waves, microwaves, infrared, visible light, UV, and radiographs (Fig. 3.1). EM radiation can be further divided into ionizing and nonionizing radiation. EM radiation at or below the UV spectrum is nonionizing, whereas radiographs are ionizing. Ionizing EM has enough energy to remove tightly bound electrons from an atom or molecule. The release of bound electrons leads to the generation of ions and free radicals. Within living cells, ions and free radicals interact with cellular machinery and cause DNA damage, which can ultimately lead to cell death. Radiotherapy uses ionizing radiation in order to cause damage to tumor cells. The photons used for therapeutic radiation generally have wavelengths of 10−11 to 10−13 m (see Fig. 3.1).1

Electromagnetic Radiation

Electromagnetic radiation was first predicted in Maxwell's equations in 1864 and its existence was demonstrated by Heinrich Hertz in 1888. During World War II, microwave technology was used in radar telecommunications. The first microwave oven was developed by the Raytheon Company of North America in 1951, demonstrating the potential of microwaves in applications that provide rapid and energy-efficient heating. In the 1970s, the microwave generator was reengineered by the Japanese into a domestic microwave oven (a simple, reliable, and cheap magnetron) to be used in food processing.

Electromagnetic radiation is widely used in food processing and can destroy microorganisms in foods. In the past few years, the microwave or radio wave region of the electromagnetic spectrum has been explored for possible use in food processing with successful results in microbial inactivation. Various food-processing methods are based on use of electromagnetic radiation energy; the most commonly used are radiofrequency, microwave, infrared, ultraviolet, visible light, and irradiation. Electromagnetic radiation is classified according to the wavelength and consequently the depth of penetration into the food. Traditionally, the nonthermal effects of the application of electromagnetic radiation refer to lethal effects without a significant rise in temperature as in the case of ionizing radiation. One of the effects of such quantum energy is the breaking of chemical bonds. Roughly one electron volt of energy is required to break a covalent bond from a molecule to produce one ion pair, and this is referred to as a nonthermal effect. Electromagnetic radiation above 2500 × 106 MHz is mostly referred to as ionizing radiation. The ionizing radiation source could be an electron beam, x-rays (machine generated), or gamma rays (from Cobalt-60 or Cesium-137), and the energy of a gamma ray is above 2 × 10−14 J. If the wavelength of radiation increases, the frequency and the energy of radiation decrease. Thus, nonionizing radiation energy is not capable of breaking all the chemical bonds. Microwaves belong to the group of nonionizing forms of radiation. Thus, they do not have sufficient energy (2 × 10−24 – 2 × 10−22 J) to affect all chemical bonds. Therefore, the nonionizing radiation is the electromagnetic radiation that does not carry enough energy or quanta to ionize atoms or molecules, represented mainly by ultraviolet rays (UV-A, UV-B, and UV-C), visible light, microwaves, and infrared.

EMR and Heat Effects: Irreversible?

EMR has a negative effect on testicular function through the creation of reactive molecules, or oxidative stress, and the concomitant disruption of the testicular ability to detoxify those molecules. Interestingly, few studies have evaluated the possible protective role of vitamins C and E in the EMR-induced oxidative stress in the testes, which may involve facilitating the restoration of normal testicular tissue and function (Gorpinchenko et al., 2014).

We know that testicular tissue is highly sensitive to ionizing radiation. Doses in the range of 0.11–15 Gy may temporarily reduce sperm counts, while 2 Gy may result in long-lasting or permanent absence normal sperm and sterility (Rowley et al., 1974). At higher doses (> 15 Gy), the Leydig cells, in charge of production and secretion of testosterone, may be affected. In addition, whole body irradiation can damage the regulation of hormones in the Brain, affecting the reproductive capability. However, more studies are necessary to evaluate if the additive effects of EMR-emitting devices and nonionizing radiation in sperm counts are reversible or irreversible. None of the above-mentioned studies have evaluated prospectively the influence of EMR irradiation on paternity. Such investigations require long-term design and many previously healthy volunteers. At the present time nobody knows the real intercommunication between the constant direct mobile phones, computers, laptops, and Wi-Fi-emitted irradiation effects on testes and developmental delays in the offspring.

Electromagnetic Radiation

Electromagnetic radiation is a form of energy that propagates as both electrical and magnetic waves traveling in packets of energy called photons. There is a spectrum of electromagnetic radiation with variable wavelengths and frequency, which in turn imparts different characteristics. Examples of energy within the electromagnetic spectrum include x-rays (most widely clinically used to treat malignant lesions), visible light, infrared light, and radio waves (Fig. 13.2).

The energy of electromagnetic radiation is quantified by an electron volt (eV), where 1 eV describes the energy gained by an electron as it is accelerated through a potential difference of 1 volt. Electromagnetic radiation deposits energy in two forms as it passes through biologic material: excitation and ionization. Excitation describes the deposition of enough energy to raise an electron to a higher electron shell without ejection of the electron. However, ionizing electromagnetic radiation has enough energy to eject one or more electrons from the atom. X-rays and gamma rays, whose properties are equivalent, are clinically the most important form of ionizing electromagnetic radiation in the treatment of cancer.

Intranuclear production of ionizing radiation occurs when unstable atoms decay to more stable nuclides via beta minus decay, beta plus decay, or electron capture. These “gamma rays” are often produced from elemental sources that are used clinically for head and neck brachytherapy. Brachytherapy describes any procedure in which an elemental source of radiation is implanted or placed in close proximity to achieve therapeutic radiation delivery to a target. Teletherapy is the use of ionizing radiation delivered with no direct contact with the patient. Historically, teletherapy machines housed a source of cobalt-60; however, due to higher skin dose and the necessity to replace the source as it decayed, teletherapy machines have transitioned to extranuclear production of ionizing radiation (i.e., x-rays) using linear accelerators (linacs) (Fig. 13.3). A linac is a high-voltage electrical device that accelerates electrons to very high energies and aims them into a target in the linac gantry head, usually containing tungsten or gold. During the collision process, the kinetic energy of the electrons is converted to high-energy photons. Therapeutic external beam radiation x-ray energies generally range between kilo- and mega-electron volts (keV or MeV). The x-ray beams are narrowed and shaped with a collimator in the treatment machine head before delivery to the patient.

Electromagnetic Radiation and Light

Electromagnetic radiation is a traveling disturbance in space that comprises electric and magnetic components. Unlike the field associated with a common magnet or the earth, the magnetic field associated with electromagnetic radiation is constantly changing its direction and strength. The electric field component undergoes exactly the same behavior. Figure 1 illustrates what would be observed if it was possible to see the electric and magnetic components of a ray of radiation as it passes. It is the interaction of the electric and magnetic components of radiation with matter that are responsible for all the phenomena that are associated with radiation, such as human vision, sunburn, microwave cookery, radio and television, and of course spectroscopy.

There are some important fundamental features of electromagnetic radiation. The periodicity of the disturbance, or the distance between adjacent troughs or adjacent peaks, is referred to as the wavelength of the radiation. The number of times the electromagnetic field undergoes a complete cycle per unit time is called the frequency. The speed with which radiation can travel is limited, and moreover, radiation of all wavelengths travel at a single speed. This is referred to as the speed of light, universally denoted by the symbol c. The speed of light (and every other radiation) is dependent on the substance through which it travels, and reaches a maximum in a vacuum. The speed of light is a constant of proportionality between frequency and wavelength as described by the following equation:

c=νλ

where ν is the radiation frequency, λ is the radiation wavelength, and c is the speed of light.

The interpretation of this equation is that wavelength and frequency are related. Electromagnetic radiation of long wavelength must have a low frequency, whereas radiation of short wavelength must have a high frequency. At the long-wavelength (low-frequency) end of the electromagnetic radiation spectrum are radio waves, while at the other end is high-frequency (short-wavelength) radiation such as gamma rays. Between these extremes lie other forms of radiation, including microwaves, infrared (IR), ultraviolet (UV), and the narrow band of radiation that can be directly observed by humans, visible light.

Radiation carries energy with it, a phenomenon that is made use of in the domestic microwave oven, for example. The radiation is carried in small discrete quantities or ‘quanta’ (singular: ‘quantum’). The amount of energy carried in each quantum is proportional to the frequency of the radiation. As frequency and wavelength have an inversely proportional relationship, the energy quantum carried is inversely proportional to wavelength. Radiation of high frequency and short wavelength (e.g., x-rays, gamma rays), therefore, is of high energy, whereas low-frequency radiation that must have long wavelength (e.g., radio waves) carries little energy.

Electromagnetic Radiation

Electromagnetic radiation (x-rays and gamma rays) is indirectly ionizing. These types of radiation do not themselves produce chemical and biologic damage, but when they are absorbed in the medium through which they pass, they give up their energy to produce fast-moving electrons by the Compton, photoelectric, or pair-production processes (Fig. 1-1). X-rays and gamma rays are forms of electromagnetic radiation that do not differ in nature or properties; the designation of x or gamma reflects the way in which they are produced. X-rays are produced extranuclearly, which means that they are generated in an electric device that accelerates electrons to high energy and then stops them abruptly in a target, made usually of tungsten or gold. Part of the kinetic energy, or energy of motion of the electrons, is converted into photons of x-rays. Gamma rays are produced intranuclearly (i.e., emitted by radioactive isotopes); they represent excess energy that is given off as the unstable nucleus breaks up and decays in its efforts to reach a stable form.

Electromagnetic Radiation

EMR is energy propagated through space at a fixed velocity, and EMR comprises a continuous spectrum of varying energies, ranging from gamma rays to radio waves. By convention, the terms photochemistry and photobiology are restricted to the effects of nonionizing radiation, including UVR, visible light, and infrared radiation, but omitting ionizing gamma rays and X-rays (Shea and Parrish, 1991). EMR can be modeled as a stream of discrete particles or packages of energy (quanta or photons), or with equal validity as waves of oscillating electrical and magnetic fields oriented transversely to each other and to the direction of propagation. Accordingly, EMR can be described by its quantum properties (energy per photon in electron volts) or wave properties (wavelength and frequency). The photobiology of skin is best understood with reference to both energy per photon and total energy striking the skin (usually expressed in joules per cm2).

Photobiologists subdivide UVR into discrete wave bands. Vacuum UVR (100–200 nm) is efficiently absorbed by the molecular nitrogen in air and therefore has little relevance to terrestrial photobiology. UVC (200–280 nm) is efficiently absorbed by the ozone layer of the stratosphere and therefore does not penetrate to the earth's surface; further deterioration of the ozone layer because of pollution may result in a biologically significant increase in the terrestrial UVR in the 280- to 290-nm wave band. Solar UVB (280–315 nm) does efficiently penetrate to the earth's surface and is responsible for most of the acute effects of sunlight on human skin, for example, erythema and delayed pigmentation. UVA (315–400 nm), also known as black light, is about 1000 times less efficient on a photon basis than UVB at inducing most of the acute photobiologic effects on skin. In addition to Sun, artificial sources of UVC, UVB, and UVA are increasingly significant for the health and appearance of human skin. More recently, intense UVA-emitting lamps have been widely used in tanning salons, under the assumption that, if UVA does not cause sunburn as effectively as UVB or UVC, it is considered to be ‘safe.’ Recently, however, UVA has been found to damage the skin by causing premature aging, nonmelanoma skin cancer, and melanoma (Ming et al., 2011a; Noonan et al., 2012).

Electromagnetic Radiation

Electromagnetic radiation is energy transmitted at a fixed velocity through sinusoidally varying electric and magnetic fields. The frequency of variation of this energy, represented by the Greek letter ν, is the number of oscillations per second, measured in hertz (Hz). The wavelength, λ, is the distance in meters between two crests of the sine wave. The velocity of propagation of the radiation is the product of the wavelength and the frequency, which, in a vacuum, is equal to the speed of light, c = 3 × 108 m/sec.

The range of wavelengths encountered in conventional physics is from 105 m for AM radio waves, to 10−7 m for visible light, to 10−12 m for x-rays and cosmic rays. Although electromagnetic radiation is conventionally described as waves of energy, quantum physics tells us that it is equally valid to describe the radiation as particle-like packets of energy called photons. Experiments that scatter x-rays off particles have been used to validate this concept. In general, the shorter wavelength radiation is more “particle-like” than the longer wavelength radiation. The energy of a photon is directly proportional to the frequency of the radiation, with a constant of proportionality called Planck's constant. That is, E = hν, where h = 6.626 × 10−34 J/s and the energy is in Joules.

Wave aspects

Electromagnetic radiation can also be regarded as sinusoidally varying electric and magnetic fields, travelling with velocity c when in vacuo. They are transverse waves: the electric and magnetic field vectors point at right angles to each other and to the direction of travel of the wave.

At any point, the graph of field strength against time is a sine wave, depicted as a solid curve in Figure 1.2a. The peak field strength is called the amplitude (A). The interval between successive crests of the wave is called the period (T). The frequency (f) is the number of crests passing a point in a second, and f = 1/T. The dashed curve refers to a later instant, showing how the wave has travelled forwards with velocity c.

At any instant, the graph of field strength against distance is also a sine wave, as shown in Figure 1.2b. The distance between successive crests of the wave is called the wavelength (λ).

Wavelength and frequency are inversely proportional to each other:

wavelength × frequency = constant.

Their product is equal to the velocity (λf = c). This relation is true of all kinds of wave motion, including sound, although for sound the velocity is about a million times less.

The types of electromagnetic radiation are listed in Table 1.3, in order of increasing photon energy, increasing frequency, and decreasing wavelength. The values are rounded and the boundaries between the types of radiation are not well defined, other than for visible light, for which the boundaries are defined by the properties of the receptor, i.e. the human eye. The nomenclature that is most commonly used in practice is emphasized in bold in the table.