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Chapter 2 The Properties of Electromagnetic Radiation

BASICS OF RADIO ASTRONOMY9 The Properties of Electromagnetic RadiationChapter 2 The Properties of Electromagnetic RadiationObjectives:When you have completed this Chapter , you will be able to define the term Electromagnetic spectrum , explain the relationship between frequency andwavelength, define amplitude, and give the relationship between energy receivedand distance from the source. You will be able to describe the limits of the S-band and X-band of the Electromagnetic spectrum . You will be able todescribe wave is Electromagnetic Radiation ?Field is a physics term for a region that is under the influence of some force that can act on matterwithin that region.

But the electromagnetic spectrum has no upper or lower limit of frequencies. It certainly has a much broader range of frequencies than the human eye can detect. In order of increasing frequency (and decreasing wavelength), the electromagnetic spectrum includes radio frequency (RF), infrared (IR, meaning “below red”), visible light,

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Transcription of Chapter 2 The Properties of Electromagnetic Radiation

1 BASICS OF RADIO ASTRONOMY9 The Properties of Electromagnetic RadiationChapter 2 The Properties of Electromagnetic RadiationObjectives:When you have completed this Chapter , you will be able to define the term Electromagnetic spectrum , explain the relationship between frequency andwavelength, define amplitude, and give the relationship between energy receivedand distance from the source. You will be able to describe the limits of the S-band and X-band of the Electromagnetic spectrum . You will be able todescribe wave is Electromagnetic Radiation ?Field is a physics term for a region that is under the influence of some force that can act on matterwithin that region.

2 For example, the Sun produces a gravitational field that attracts the planets inthe solar system and thus influences their electric charges produce electric fields, whereas moving electric charges produce bothelectric and magnetic fields. Regularly repeating changes in these fields produce what we callelectromagnetic Radiation . Electromagnetic Radiation transports energy from point to point. Thisradiation propagates (moves) through space at 299,792 km per second (about 186,000 miles persecond). That is, it travels at the speed of light. Indeed light is just one form of other forms of Electromagnetic Radiation are X-rays, microwaves, infrared Radiation , AMand FM radio waves, and ultraviolet Radiation .

3 The Properties of Electromagnetic radiationdepend strongly on its frequency. Frequency is the rate at which the radiating electromagneticfield is oscillating. Frequencies of Electromagnetic Radiation are given in Hertz (Hz), named forHeinrich Hertz (1857-1894), the first person to generate radio waves. One Hertz is one cycle and WavelengthAs the Radiation propagates at a given frequency, it has an associated wavelength that is, thedistance between successive crests or successive troughs. Wavelengths are generally given inmeters (or some decimal fraction of a meter) or Angstroms ( , 10-10 meter).Since all Electromagnetic Radiation travels at the same speed (in a vacuum), the number of crests(or troughs) passing a given point in space in a given unit of time (say, one second), varies withthe wavelength.

4 For example, 10 waves of wavelength 10 meters will pass by a point in the sameJPL D-1383510length of time it would take 1 wave of wavelength 100 meters. Since all forms of electromag-netic energy travel at the speed of light, the wavelength equals the speed of light divided by thefrequency of oscillation (moving from crest to crest or trough to trough).In the drawing below, Electromagnetic waves are passing point B, moving to the right at the speedof light (usually represented as c, and given in km/sec). If we measure to the left of B a distanceD equal to the distance light travels in one second ( x 105 km), we arrive at point A alongthe wave train that will just pass point B after a period of 1 second (moving left to right).

5 Thefrequency f of the wave train that is, the number of waves between A and B times the lengthof each, , equals the distance D traveled in one we talk about the frequency of Electromagnetic Radiation in terms of oscillations persecond and the speed of light in terms of distance travelled per second, we can saySpeed of light = Wavelength x FrequencyWavelength = Speed of light FrequencyFrequency = Speed of light Wavelengthorc = fAmplitudeThe maximum variation in the strength of an Electromagnetic wave in one wavelength iscalled its amplitude. In other words, amplitude is the height from the crest to the troughof the of Wavelength and Frequencyof Electromagnetic WavesDirection of wave propagation ABDBASICS OF RADIO ASTRONOMY11 Inverse-Square Law of PropagationAs Electromagnetic Radiation leaves its source, it spreads out, traveling in straight lines, as if itwere covering the surface of an ever expanding sphere.

6 This area increases proportionally to thesquare of the distance the Radiation has traveled. In other words, the area of this expandingsphere is calculated as 4 R2 , where R is the distance the Radiation has travelled, that is, theradius of the expanding sphere. This relationship is known as the inverse-square law of (electro-magnetic) propagation. It accounts for loss of signal strength over space, called space loss. Forexample, Saturn is approximately 10 times farther from the sun than is Earth. (Earth to sundistance is defined as one astronomical unit, AU). By the time the sun s Radiation reaches Saturn,it is spread over 100 times the area it covers at one AU.

7 Thus, Saturn receives only 1/100th thesolar energy flux (that is, energy per unit area) that Earth inverse-square law is significant to the exploration of the universe. It means that the concen-tration of Electromagnetic Radiation decreases very rapidly with increasing distance from theemitter. Whether the emitter is a spacecraft with a low-power transmitter, an extremely powerfulstar, or a radio galaxy, because of the great distances and the small area that Earth covers on thehuge imaginary sphere formed by the radius of the expanding energy, it will deliver only a smallamount of energy to a detector on Square Law of Electromagnetic RadiationThe Properties of Electromagnetic RadiationJPL D-1383512 The Electromagnetic SpectrumLight is Electromagnetic Radiation at those frequencies to which human eyes (and those of mostother sighted species) happen to be sensitive.

8 But the Electromagnetic spectrum has no upper orlower limit of frequencies. It certainly has a much broader range of frequencies than the humaneye can detect. In order of increasing frequency (and decreasing wavelength), the electromagneticspectrum includes radio frequency (RF), infrared (IR, meaning below red ), visible light,ultraviolet (UV, meaning above violet ), X-rays, and gamma rays. These designations describeonly different frequencies of the same phenomenon: Electromagnetic frequencies shown in the following two diagrams are within range of those generated bycommon sources and observable using common detectors.

9 Ranges such as microwaves, infrared ,etc., overlap. They are categorized in spectrum charts by the artificial techniques we use toproduce OF RADIO ASTRONOMY1310-4 nm 10-2 nm 1 nm 102 nm 104 nm 1 mm = 106 nm 10 cm = 108 nm 10 m = 1010 nm 1 km = 1012 nm 400 nm 700 nm Gamma rays X-rays Ultraviolet infrared Radio waves Visible light Wavelength (nanometers) Electromagnetic spectrum :Visible light only a fraction of the spectrum (red) (violet) The Properties of Electromagnetic RadiationJPL D-1383514100 kilometers (103 m)10 Examples3 kilohertz (103 Hz)30 kilohertz (103 Hz)300 kilohertz (103 Hz)3 Megahertz (106 Hz)30 Megahertz (106 Hz)300 Megahertz (106 Hz)3 Gigahertz (109 Hz)30 Gigahertz (109 Hz)300 Gigahertz (109 Hz)3 Terahertz (1012 Hz)30 Terahertz (1012 Hz)300 Terahertz (1012 Hz)3 Petahertz (1015 Hz)30 Petahertz (1015 Hz)300 Petahertz (1015 Hz)3 Exahertz (1018 Hz)30 Exahertz (1018 Hz)300 Exahertz (1018 Hz)

10 3 X 1021 Hz30 X 1021 Hz300 X 1021 Hz3 X 1024 Hz30 X 1024 Hz300 X 1024 Hz FrequencyWavelengthRadio Frequencies (RF)The Electromagnetic spectrum :Wavelength/frequency chart1100 Meters10 1100 millimeters (10-3 m)10 1100 micrometers (10-6 m)10 1100 nanometers (10-9 m)10 1100 picometers (10-12 m)10 1100 femtometers (10-15 m)101100 attometers (10-18 m)10 1 Long Radio WavesAMShort WavesFootball FieldHumanFM & TVRadarS-bandX-bandInfrared RadiationVisible LightUltraviolet RadiationX-raysGamma RaysAtomic NucleusAtomsVirusBacteriumGrains of Sand Longer WavelengthsShorter WavelengthsLower FrequenciesHigher FrequenciesExamplesBASICS OF RADIO ASTRONOMY15 Electromagnetic Radiation with frequencies between about 5 kHz and 300 GHz is referred to asradio frequency (RF)


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