Electromagnetic radiation


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Electromagnetic radiation

  1. 1. ELECTROMAGNETIC RADIATION (EMR) 1 Electromagnetic radiation (EMR)Electromagnetic radiation (EM radiation or EMR) is a form of energy emitted and absorbed by chargedparticles, which exhibits wave-like behavior as it travels through space. EMR hasboth electric and magnetic field components, EMR carries energy—sometimes called radiant energy—through space continuously away from the source (this is not true of the near-field part of the EMfield). EMR also carries both momentum and angular momentum. These properties may all beimparted to matter with which it interacts. EMR is produced from other types of energy when created,and it is converted to other types of energy when it is destroyed. The photon is the quantum of theelectromagnetic interaction, and is the basic "unit" or constituent of all forms of EMR. The quantumnature of light becomes more apparent at high frequencies (or high photon energy). Such photonsbehave more like particles than lower-frequency photons do.In simple words,Electromagnetic radiation refers to the transfer of energy through electromagneticwaves. These waves can transfer energy by traveling in a vacuum, and (in a vacuum) they travel ataround,v = 3.00*10^8m/sYou may have identified this speed as the speed of light. Indeed, light is electromagnetic radiation.Visible light forms a small part of the electromagnetic spectrum, a graphical representation of therelationship between frequency and wavelength in electromagnetic waves.Electromagnetic waves include radio waves, infrared, visible light, and gamma rays (there are a fewmore). They all differ in frequency and wavelength. The speed of a wave is given by the equation,velocity = (wavelength) x (frequency)As you can see, wavelength and frequency are inversely proportional. That means that as onedecreases, the other increases (given, of course, a constant velocity, which has the magnitude Imentioned before). Radio waves have the longest wavelength (and thus the less frequency).Characteristics of Electromagnetic Radiation.Electromagnetic energy or radiation is the medium for transmitting information from an object tosensor. The information is propagated by electromagnetic radiation at the velocity of light from thesource (earth surface) directly through free space or indirectly by reflection, scattering and radiation tothe sensor. The electromagnetic radiation is looked at as sinusoidal waves which are composed of acombination of two fields. An electric field (which we will use, in this course, to explain absorption MICHAEL HEMBROM
  2. 2. ELECTROMAGNETIC RADIATION (EMR) 2and emission of radiation by analyses) and a magnetic field at right angle to the electric field (whichwill be used to explain phenomena like nuclear magnetic resonance in the course of special topics inanalytical chemistry offered to Chemistry students only). However, if we look at the models describingelectromagnetic radiation, we can present the classical wave model and the quantum mechanicalparticle model.The classical wave model describes electromagnetic radiation as waves that have a wavelength,frequency, velocity, and amplitude. These properties of electromagnetic radiation can explain classicalcharacteristics of electromagnetic radiation like reflection, refraction, diffraction, interference, etc.However, the wave model cannot explain the phenomena of absorption and emission of radiation,which leads to the quantum mechanical model.An electromagnetic beam can be described as sinusoidal electric fields at right angle to magnetic fields.If we look at one wave only at a certain plain (monochromatic plane polarized wave) we can picturethe wave as follows:We will only deal with the electric field of the electromagnetic radiation and will thus refer to anelectromagnetic wave as an electric field having the shape of a sinusoidal wave. The arrows in thefigure above represent few electric vectors while the yellow solid sinusoidal wave is the magnetic fieldassociated to the electric field of the wave. MICHAEL HEMBROM
  3. 3. ELECTROMAGNETIC RADIATION (EMR) 3Wave ParametersThere are some parameters which can describe the wave. These include the followinf:1. Wavelength (l)The wavelength of a wave is the distance between two consecutive maxima or two consecutive minimaon the wave. It can also be defined as the distance between two equivalent points on two successivemaxima or minima. This can be seen on the figure below:2. Ammplitude (A)The amplitude of the wave is represented by the length of the electrical vector at a maximum orminimum in the wave. In the figure above, the amplitude is the length of any of the vertical arrowsperpendicular to the direction of propagation of the wave. MICHAEL HEMBROM
  4. 4. ELECTROMAGNETIC RADIATION (EMR) 43. Frequency (n)The frequency of the wave is directly proportional to the energy of the wave and is defined as thenumber of wavelengths passing a fixed point in space in one second.4. Period (p)The period of the wave is the time in seconds required for one wavelength to pass a fixed point inspace.5. Velocity (v)The velocity of a wave is defined as the multiplication of the frequency times the wavelength. Thismeans:V=nlThe velocity of light in vacuum is greater than its velocity in any other medium. Since the frequency ofthe wave is a constant and is a property of the source, the decrease in velocity of electromagneticradiation in media other than vacuum should thus be attributed to a decrease in the wavelength ofradiation upon passage through that medium. MICHAEL HEMBROM
  5. 5. ELECTROMAGNETIC RADIATION (EMR) 56. Wavenumber (n)The reciprocal of wavelength in centimeters is called the wave number. This is an important propertyespecially in the study of infrared spectroscopy.n=kn7. Radiation Power and IntensityThe power of the radiation is related to the square of the amplitude. The intensity is also related to thesquare of the amplitude. However, the power and intensity have different concepts but will be usedsynonymously in this course. MICHAEL HEMBROM
  6. 6. ELECTROMAGNETIC RADIATION (EMR) 6ELECTROMAGNETIC SPECTRUMThe electromagnetic radiation covers a vast spectrum of frequencies and wavelengths. This includesthe very energetic gamma-rays radiation with a wavelength range from 0.005 – 1.4 Ao to radio waves raysin the wavelength range up to meters (exceedingly low energy). However, the region of interest to us inthis course is rather a very limited range from 180 780 nm. This limited range covers both ultraviolet ry 180-780and visible radiation. It is of interest to indicate at this point that each region of electromagneticspectrum requires a special set of instrumental components. This can therefore make it clear for us that thereforethe type of instrumental components we will deal with will be extremely limited but, at the same time,will be representative enough.In general, EM radiation (the designation radiation excludes static electric and magnetic and near afields) is classified by wavelength into radio, microwave, infrared, the visible spectrum we perceive as )visible light, ultraviolet, X-rays and gamma rays. he behavior of EM radiation depends on its rays, .frequency. Lower frequencies have longer wavelengths, and higher frequencies have shorterwavelengths, and are associated with photons of higher energy.Wavelength region and their specificationThe types of electromagnetic radiation are broadly classified into the following classes: 1. Gamma radiation 2. X-ray radiation 3. Ultraviolet radiation 4. Visible radiation 5. Infrared radiation 6. Microwave radiation 7. Radio waves MICHAEL HEMBROM
  7. 7. ELECTROMAGNETIC RADIATION (EMR) 7Gamma raysdiscovered by Paul Villard in 1900. Radioactive materials (some natural and others made by man inthings like nuclear power plants) can emit gamma-rays. Big particle accelerators that scientists use tohelp them understand what matter is made of can sometimes generate gamma-rays. But the biggestgamma-ray generator of all is the Universe! It makes gamma radiation in all kinds of ways.Wavelength rangest between 10-11-10-8.X-raysAfter UV come X-rays, which, like the upper ranges of UV are also ionizing. However, due to theirhigher energies, X-rays can also interact with matter by means of the Compton effect. Hard X-rayshave shorter wavelengths than soft X-rays. As they can pass through most substances, X-rays can beused to see through objects, wavelength ranges between 0.3x10-8cm – 3x10-6cm.Ultraviolet lightThe Sun is a source of ultraviolet (or UV) radiation, because it is the UV rays that cause our skin toburn! Stars and other "hot" objects in space emit UV radiation.The wavelength of UV rays is shorterthan the violet end of the visible spectrum but longer than the X-ray.UV in the very shortest range (next to X-rays) is capable even of ionizing atoms (see photoelectriceffect), greatly changing their physical behavior. Wavelength ranges between 0.3 µ-0.4 µ.Visible radiation (light)his is the part that our eyes see. Visible radiation is emitted by everything from fireflies to light bulbs tostars ... also by fast-moving particles hitting other particles. This is the range in which the sun andother stars emit most of their radiation and the spectrum that the human eye is the most sensitive to.Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules andatoms that move from one energy level to another. The light we see with our eyes is really a very smallportion of the electromagnetic spectrum. MICHAEL HEMBROM
  8. 8. ELECTROMAGNETIC RADIATION (EMR) 8Visible radiation has three types. 1. Blue-0.4 µ -0.5 µ. 2. Green- 0.5 µ-0.6 µ. 3. Red- 0.6 µ-0.7 µInfrared radiationThe infrared part of the electromagnetic spectrum covers the range from roughly 0.7 µ to 1 µ. It can bedivided into three parts: 1. near-infrared -0.7 µ-1.3 µ 2. Mid infrared -1.3 µ-3.0 µ 3. Far or thermal infrared- 3.0 µ-0.3mmMicrowave radiationMicrowaves are waves that are typically short enough to employ tubular metal waveguides ofreasonable diameter. Microwave energy is produced with klystron and magnetron tubes, and with solidstate diodes such as Gunn and IMPATT devices.wavelength ranges between 1mm-1m.RadiowaveRadio waves generally are utilized by antennas of appropriate size (according to the principleof resonance), with wavelengths ranging from hundreds of meters to about one millimeter. They areused for transmission of data, via modulation. Television, mobile phones, wireless networking,and amateur radio all use radio waves. The use of the radio spectrum is regulated by manygovernments through frequency allocation. Wavelength ranges between 30cm – 3km. MICHAEL HEMBROM