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Objectives • Describe what waves and more specifically, electromagnetic waves are and how they are produced. • Recognize that electricity and magnetism are two aspects of a single electromagnetic force. • Explain how electromagnetic waves transfer energy. • Describe various applications of electromagnetic waves. Waves Direction A wave is a pattern that repeats itself in a cycle of both time and space. Waves are characterized by the velocity with which they move, their frequency, and their wavelength. Waves wave period • The amount of time required for a wave to repeat itself at a specific point in space frequency • The number of wave crests passing any given point per unit of time. Waves wavelength The length from one point on a wave to the point where it is repeated exactly in space, at a given time. The wave speed is simply the product of the wavelength and the frequency: Waves diffraction The tendency of waves to bend around corners. The diffraction of light establishes its nature as a wave. interference The ability of two or more waves to interact in such a way that they either reinforce or cancel each other. Waves • If radiation were composed of rays or particles moving in perfectly straight lines, no bending would occur • the outline of the hole becomes "fuzzy" due to bending from diffraction Waves Propagation of Electromagnetic Waves • Electromagnetic waves are comprised of oscillating, perpendicular electric and magnetic fields. • They travel at the speed of light. • Electromagnetic waves are transverse waves; that is, the direction of travel is perpendicular to the the direction of oscillating electric and magnetic fields. Electromagnetic Waves Propagation of Electromagnetic Waves • All electromagnetic waves are produced by accelerating charges. • Electromagnetic waves transfer energy. The energy of electromagnetic waves is stored in the waves’ oscillating electric and magnetic fields. • Electromagnetic radiation is the transfer of energy associated with an electric and magnetic field. Electromagnetic radiation varies periodically and travels at the speed of light. It can be in a vacuum. The Sun at Different Wavelengths of Radiation Propagation of Electromagnetic Waves • High-energy electromagnetic waves behave like particles. • An electromagnetic wave’s frequency makes the wave behave more like a particle. This notion is called the wave-particle duality. • A photon is a unit of light. • Photons can be thought of as particles of electromagnetic radiation that have zero mass and carry one quantum of energy. The Electromagnetic Spectrum • The electromagnetic spectrum ranges from very long radio waves to very short-wavelength gamma waves. • The electromagnetic spectrum has a wide variety of applications and characteristics that cover a broad range of wavelengths and frequencies. The Electromagnetic Spectrum • Radio Waves – longest wavelengths – communications, tv • Microwaves – 30 cm to 1 mm – radar, cell phones • Infrared – 1 mm to 700 nm – heat, photography • Visible light – 700 nm (red) to 400 nm (violet) • Ultraviolet – 400 nm to 60 nm – disinfection, spectroscopy • X rays – 60 nm to 10–4 nm – medicine, astronomy, security screening • Gamma Rays – less than 0.1 nm – cancer treatment, astronomy The Electromagnetic Spectrum The Electromagnetic Spectrum visible spectrum • The small range of the electromagnetic spectrum that human eyes perceive as light. The visible spectrum ranges from about 4000 to 7000 angstroms, corresponding to blue through red light. The Electromagnetic Spectrum Note that wave frequency (in hertz) increases from left to right, and wavelength (in meters) increases from right to left. These wave properties behave in opposite ways because, as noted earlier, they are inversely related. The Electromagnetic Spectrum • Our eyes are sensitive to only a minute portion of the many different kinds of radiation known. • Only a small fraction of the radiation produced by astronomical objects actually reaches our eyes, in part because of the opacity of Earth's atmosphere. The Electromagnetic Spectrum • Opacity is the extent to which radiation is blocked by the material through which it is passing—in this case, air. • The more opaque an object is, the less radiation gets through it. • Opacity is the opposite of transparency. The Electromagnetic Spectrum The Electromagnetic Spectrum • What causes opacity to vary along the spectrum? Certain atmospheric gases are known to absorb radiation very efficiently at some wavelengths. For example, water vapor (H2O) and oxygen (O2) absorb radio waves having wavelengths less than about a centimeter, whereas water vapor and carbon dioxide (CO2) are strong absorbers of infrared radiation. Ultraviolet, X-ray, and gamma-ray radiation are completely blocked by the ozone layer high in Earth's atmosphere. The Electromagnetic Spectrum • White light is a mixture of colors, which we conventionally divide into six major hues—red, orange, yellow, green, blue, and violet. • In principle, the original beam of white light could be restored by passing the entire red-to-violet range of colors—called a spectrum—through a second, oppositely oriented prism to recombine the colored beams. • This experiment was first reported by Isaac Newton over 300 years ago. The Electromagnetic Spectrum • While passing through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit narrows the beam of radiation. The image on the screen is just a series of different-colored images of the slit. The Electromagnetic Spectrum • Astronomers often use a unit called the nanometer (nm) when describing the wavelength of light (see Appendix 2). • There are 109 nanometers in 1 meter. An older unit called the angstrom (1Å - 10 -10 m - 0.1 nm) is also widely used. • The unit is named after the nineteenth-century Swedish physicist Anders Ångstrom— pronounced "ongstrem.“ • In SI units, the nanometer is preferred. Thus, the visible spectrum covers the wavelength range from 400 to 700 nm (4000 to 7000 Å). Blackbody Radiation intensity A basic property of electromagnetic radiation that specifies the amount or strength of the radiation. Often used to specify the amount or strength of radiation at any point in space. Blackbody Radiation •No natural object emits all its radiation at just one frequency. •Energy is generally spread out over a range of frequencies. •We look at the way in which the intensity of this radiation is distributed across the electromagnetic spectrum, Blackbody Radiation The blackbody, or Planck, curve represents the distribution of the intensity of radiation emitted by any heated object. Blackbody Radiation blackbody curve The characteristic way in which the intensity of radiation emitted by a hot object depends on frequency. The frequency at which the emitted intensity is highest is an indication of the temperature of the radiating object. Also referred to as the Planck curve. Blackbody Radiation As an object is heated the radiation it emits peaks at higher and higher frequencies. Shown here are curves corresponding to temperatures of 300 K (room temperature), 1000 K (beginning to glow deep red), 4000 K (red hot), and 7000 K (white hot). Blackbody Radiation As the temperature continues to rise, the peak of the metal's blackbody curve moves through the visible spectrum, from red (the 4000 K curve) through yellow. The metal eventually becomes white hot when its blackbody curve peaks in the blue or violet part of the spectrum (the 7000 K curve), However, the low-frequency tail of the curve extends through the entire visible spectrum so white light is emitted. Blackbody Radiation Many extraterrestrial objects, however, do emit copious quantities of ultraviolet, X-ray, and even gamma-ray radiation. Although most sunlight is visible, a great deal of information about our parent star can be obtained by studying it in other parts of the electromagnetic spectrum. Blackbody Radiation Four images of the Sun, made using (a) visible light, (b) ultraviolet light, (c) X-rays, and (d) radio waves. By studying the similarities and differences among these views of the same object, astronomers can find important clues to its structure and composition Blackbody Radiation Other cosmic objects have surfaces very much cooler or hotter than the Sun's, emitting most of their radiation in invisible parts of the spectrum a) A cool, invisible galactic gas cloud called Rho Ophiuchi. At a temperature of 60 K, it emits mostly low-frequency radio radiation. (b) A dim, young star (shown here in red) near the center of the Orion Nebula. The star's atmosphere, at 600 K, radiates primarily in the infrared. c) The Sun's surface, at 6000 K, is brightest in the visible region of the electromagnetic spectrum. d) A cluster of very bright stars, called Omega Centauri, as observed by a telescope aboard a space shuttle. At a temperature of 60,000 K, these stars radiate strongly in the ultraviolet. Radio Astronomy radio telescope Large instrument designed to detect radiation from space in radio wavelengths. Radio Astronomy •The radio window in the electromagnetic spectrum is much wider than the optical window.. • Atmosphere is no hindrance to long-wavelength radiation, radio astronomers have built many ground-based radio telescopes to detect cosmic radio waves. •These devices have all been constructed since the 1950s—radio astronomy is a much younger subject than optical astronomy. Radio Astronomy a fairly typical radio telescope, the large 43-m (140-foot)-diameter telescope located at the National Radio Astronomy Observatory in West Virginia. much larger than reflecting optical telescopes, most radio telescopes are built in basically the same way. They have a large, horseshoe-shaped mount supporting a huge, curved metal dish that serves as the collecting area. The dish captures cosmic radio waves and reflects them to the focus, where a receiver detects the signals and channels them to a computer. Radio Astronomy However, unlike optical instruments, which can detect all visible wavelengths simultaneously, radio detectors normally register only a narrow band of wavelengths at any one time. To observe radiation at another radio frequency, we must retune the equipment, much as we tune a television set to a different channel. Radio Astronomy An aerial photograph of the 300-m-diameter dish in Puerto Rico. The receivers that detect the focused radiation are suspended nearly 300 m above the dish. a close-up of the radio receivers hanging high above the dish. Technicians adjusting the dish surface. Radio Astronomy Despite the inherent disadvantage of relatively poor angular resolution, radio astronomy enjoys many advantages: •Radio telescopes can observe 24 hours a day. •Observations can often be made through cloudy skies, and they can detect the longest-wavelength radio waves even during rain or snowstorms •It opens up a whole new window on the universe New Universe 1. First, just as objects that are bright in the visible part of the spectrum (the Sun, for example) are not necessarily strong radio emitters, many of the strongest radio sources in the universe emit little or no visible light. 2. Second, visible light may be strongly absorbed by interstellar dust along the line of sight to a source. Radio waves, on the other hand, are generally unaffected by intervening matter. New Universe The Orion Nebula is a starforming region about 1500 light years from Earth. The bright regions in this photograph are stars and clouds of glowing gas. The dark regions are not empty, but their visible emission is obscured by interstellar matter. Superimposed on the optical image is a radio contour map of the same region. Each curve of the contour map represents a different intensity of radio emission. The resolution of the optical image is about 1"; that of the radio map 1'. Interferometry interferometer Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost telescopes. Interferometry In interferometry, two or more radio telescopes are used in tandem to observe the same object at the same wavelength and at the same time Through electronic cables or radio links, the signals received by each antenna in the array making up the interferometer are sent to a central computer that combines and stores the data. The technique works by analyzing how the waves interfere with each other when added together. Interferometry This large interferometer is made up of 27 separate dishes spread along a Y-shaped pattern about 30 km across on the Plain of San Augustin, NM. The most sensitive radio device in the world, it is called the Very Large Array or VLA, for short. (b) A close-up view from ground level of some of the VLA antennas. Notice that the dishes are mounted on railroad tracks so that they can be repositioned easily. Interferometry VLA radio "radiograph” of the spiral galaxy M51, observed at radio frequencies with an angular resolution of a few arc seconds (a) shows nearly as much detail as (b) an actual (light) photograph of that same galaxy made with the 4-m Kitt Peak optical telescope. Interferometry In 1997 a group of scientists in Cambridge, England, succeeded in combining the light from three small optical telescopes to produce a single, remarkably clear, image. Each telescope is only 0.4 m in diameter, but when the equipment is positioned 6 m apart, the resulting resolution is a stunning 0.01" Infrared Telescopes • Infrared studies are a very important component of modern observational astronomy. • Infrared telescopes resemble optical telescopes (indeed, many optical telescopes are also used for infrared work), but their detectors are sensitive to longer-wavelength radiation. Infrared Telescopes • Although most infrared radiation is absorbed by the atmosphere (primarily by water vapor), there are a few windows in the high-frequency part of the infrared spectrum where the opacity is low enough to allow ground-based observations • Some of the most useful infrared observing is done from the ground, even though the radiation is somewhat diminished in intensity by our atmosphere. • As with radio observations, the longer wavelength of infrared radiation often enables us to perceive objects partially hidden from optical view. Infrared Telescopes • An optical photograph (a) taken near San Jose, California, and an infrared photo (b) of the same area taken at the same time. •Longer-wavelength infrared radiation can penetrate smog much better than shortwavelength visible light. Infrared Telescopes (a) The Orion Nebula and its surrounding environment was made by the Infrared Astronomy Satellite. The whiter regions denote greater strength of infrared radiation; the false colors denote different temperatures, descending from white to red to black. (b) The same region photographed in visible light. The labels alpha and beta refer to Betelgeuse and Rigel, the two brightest stars in the constellation. Note how the red star Betelgeuse can be seen in the infrared (part a), but the blue star Rigel cannot. UV Telescopes ultraviolet telescope A telescope that is designed to collect radiation in the ultraviolet part of the spectrum. The Earth's atmosphere is partially opaque to these wavelengths, so ultraviolet telescopes are put on rockets, balloons, or satellites to get high above most or all of the atmosphere. The ultraviolet domain, this region of the spectrum, extends in wavelength from 400 nm (blue light) down to a few nanometers, has only recently begun to be explored. UV Telescopes Earth's atmosphere is partially opaque to radiation below 400 nm and is totally opaque below about 300 nm Rockets, balloons, or satellites are therefore essential Hubble Space Telescope best known as an optical telescope, is also a superb ultraviolet instrument. Telescopes High-energy astronomy studies the universe as it presents itself to us in X-rays and gamma rays—the types of radiation whose photons have the highest frequencies and hence the greatest energies. How do we detect radiation of such short wavelengths? Telescopes First, it must be captured high above Earth's atmosphere because none of it reaches the ground. Second, its detection requires the use of equipment basically different in design from that used to capture the relatively low energy radiation discussed up to this point. Telescopes The basic difference in the design of high-energy telescopes comes about because X and gamma rays cannot be reflected easily by any kind of surface. These rays tend to pass straight through, or be absorbed by, any material they strike. When X-rays barely graze a surface, however, they can be reflected from it in a way that yields an image, although the mirror design is fairly complex. For gamma rays no such method of producing an image has yet been devised. Present-day gamma-ray telescopes simply point in a specified direction and count photons received. Telescopes The arrangement of mirrors in an X-ray telescope allows X-rays to be reflected at grazing angles and focused to form an image.