Doppler effect in satellite communication ppt

doppler effect in radar ppt and doppler effect physics ppt
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Prof.EvanBaros,United Kingdom,Teacher
Published Date:26-07-2017
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3.5 The Doppler Effect The wavelength (or frequency) of a wave, as measured by an observer, depends on the relative radial speed of the source and observer. Radial motion means: motion towards or away; along the line of sight. The Doppler effect involves only this component of motion. What we get from measuring it is called the radial velocity. Moving away: wavelengths Moving toward: wavelengths increase (“redshift”) decrease (“blueshift”)3.5 The Doppler Effect Relationship between wavelength and speed: ⇒ Shift in λ compared to “rest (no motion) wavelength” is proportional to radial velocity ⇒ So if you know the exact wavelength where some feature in the spectrum should be (“true wavelength”), and the wavelength at which it appears (“apparent wavelength”), you can obtain the radial velocity. ⇒ This is how we get speeds of cosmic objects, stars, galaxies, even expansion of universe. Actual formula is: λ(apparent)/ λ(true) = 1 ± (speed of object/speed of light) where the ± sign means it is + if it is moving away from us (redshift, longer wavelength), - if it is moving toward us (blueshift, shorter wavelength) This applies to any wave; and no reason not to use frequency instead of wavelength. Textbook writes it this way:3.5 The Doppler Effect See textbook for this rather confusing example. In class we’ll use a simpler example of water waves in a pond. Important point: Doppler effect depends only on the relative motion of source and observerMore Precisely 3-3: Measuring Velocities with the Doppler Effect Example: For a speed of 30 km/s, the Doppler shift is given by speed of object speed of light This may seem small, but it is easily detectable with a radar gun. It is NOT so easy to detect from the spectrum of an astronomical object, unless you know something about spectral lines.Look at the Doppler shift formula again: λ(apparent)/ λ(true) = 1 ± (radial velocity of object./speed of light) If velocity of object away or toward us is much less than the speed of light (true for almost all objects in the universe), the apparent wavelength will be only slightly different from the “laboratory” or “rest” wavelength. For most objects in the universe, this relative shift is so tiny, that we can’t detect it using the “shift” of the whole continuous spectrum. But we can use places in the spectrum whose wavelengths are precisely known by the presence of spectral lines (the subject of Chapter 4)Chapter 4 Spectroscopy Don’t worry if you can’t understand what this pretty picture represents, unless it is the day before the next exam. A more important question Is why the authors insist on showing this form of spectra without adequately explaining4.1 Spectral Lines Spectroscope: Splits light into component colors (wavelengths, frequencies) Most of this illustration is completely unnecessary The only important point is that light from any object can be spread out into a “rainbow” of wavelengths. A spectrum is a picture of how much light is at each wavelength. This illustration is showing a continuous spectrum.4.1 Spectral “Lines” Continuous spectrum: Continuous range of frequencies emitted by an object (something like the “black bodies” we discussed in ch.3) Emission lines: Single frequencies emitted by particular atoms in a hot gas Absorption lines: If a continuous spectrum passes through a cool gas, atoms of the gas will absorb the same frequencies they emitSpectral lines Spectral lines—very narrow, well-defined (in wavelength) wavelength/frequency regions in the spectrum where excess photon energy appears (emission lines) or else where photons are missing (absorption lines). Cartoon view of absorption lines, both in the spectrum as a graph (below), and in the recorded spectrum (top), the band of colorsthis is just how the spectra are gatheredpay no attention to the rectangular shape Often these lines are superimposed on a smooth, “continuous” spectrum, which is the near-blackbody emission of a heated object that we have been discussing so far (ch. 3, Wien, Stefan-Boltzmann).Spectral of real astronomical objects Here are spectra of two real astronomical objects, a comet (top) and star (bottom). By the time you take the next exam, you should be able to explain why these look so different. The wavelengths, shapes, and strengths of these “spectral lines” are the keys to understanding many of the physical properties of planets, stars and galaxies.Spectra are not black rectangles with vertical lines. They are a recording of how many photons per second are being emitted as a function of wavelength4.1 Spectral Lines Emission spectrum can be used to identify elements An absorption spectrum can also be used to identify elements. These are the emission and absorption spectra of sodium:Here is the Sun’s spectrum, along with a blackbody of the sun’s temperature (topwhy are there no lines?), and the spectra of individual elements as observed in the laboratory. Each spectral line is a “chemical fingerprint” telling you which elements, and how much of each element, is contained in the object you are observing. Continuous spectrum Sun (absorption lines) Emission lines of various elements Kirchoff’s laws: don’t memorize them. Instead, come back to this illustration later and find if you understand enough to explain them “Kirchoff’s laws” are usually presented with prisms and striped colorful “spectra.” This is confusing, and they aren’t even “laws” at all Just note that “prism” is supposed to represent an instrument, called a “spectrometer.” Maybe a star, or a planet Maybe the atmosphere of a star or planet If the light from the hot “star” or blackbody doesn’t pass through any low-density gas, then the spectrum is featurelessit is a “continuous spectrum.” (top)  If that continuous spectrum passes through a cloud of gas that is cooler than the source, the cloud can absorb particular wavelengths, and you get an absorption spectrum (middle)  But if the gas is hot, say at least a few thousand degrees, it will emit spectral lines on its own (bottom), I.e. emission lines.  How does this occur? The answer lies in the structure of atoms.Next : Energy levels of electrons in atoms4.2 Atoms and Radiation Existence of spectral lines required new model of atom, so that only certain amounts of energy could be emitted or absorbed Bohr model had certain allowed orbits for electron4.2 Atoms and Radiation Emission energies correspond to energy differences between allowed levels Modern model has electron “cloud” rather than orbit4.3 The Formation of Spectral Lines Absorption spectrum: Created when atoms absorb photons of right energy for excitation Multielectron atoms: Much more complicated spectra, many more possible states Ionization changes energy levelsEnergy levels in H and HeSpectral lines = electronic transitions The light emitted has a wavelength corresponding to the the energy difference between the two electron energy levels.

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