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ANSWERS TO CHAPTER 7 Radiations and Telescopes 809. (d) Gamma ray. 810. (c) Cosmic ray. (False) 811. Positively charged atomic nuclei (protons and the nuclei of helium and heavier elements) plus a minor admixture of electrons all having high energies, characteristically several billions of electron volts. Cosmic rays are present throughout the solar system and presumably throughout the universe. The processes by which they are accelerated to such high energies are thought to occur in supernova and in interstellar space. 812. (b) Roemer. 813. In its annual orbit around the Sun, the Earth alternately moves toward Jupiter and away from it. When the Earth is approaching Jupiter, the apparent orbital period of a Jovian satellite-as measured for example by the interval between successive occultations by the planet's disc-is less than its true sidereal period. The reason is that as the Earth-Jupiter distance decreases, the 233 234 / Radiations and Telescopes light signal requires a lesser time to reach the Earth than if the Earth-Jupiter distance were constant. About six months later the situation is reversed and the apparent period is greater than the true period. The magnitude of this effect yields a measurement of the speed of light in terms of the speed of the Earth relative to Jupiter. It is, of course, desirable to cumulate the effect over many orbits in order to improve the accuracy of the determination. 814. (c) Finite speed oflight. 815. During the period of revolution of 10 (152,880 s) the Earth moves 4.59 X 106 km closer to Jupiter. An observer on the Earth notes that the interval between successive occurrences of a particular event in the orbit of 10 (e.g., occultation by the planet) is 4.59 x 106/300,000 = 15.3 s less than expected. Thus, the apparent period of revolution of 10 is less than its true value by this amount. The effect is of opposite algebraic sign about six months later (d. answer to problem 813). 816. 300,000 km s-1 (= 186,000 miles s-1 ). 817.• The straightforward conceptual method for measuring the speed of light c is as follows. Emit a brief pulse of light at the focal point of a lens or mirror so that the resulting parallel beam is directed to a plane mirror at distance D. Then by means of a photocell, detect the reflected pulse, measure the lapse of time ~t between emission and return of the pulse, and calculate the ratio c = 2D/~t. The experimental difficulty of making a precision determination of c by this method is suggested by the fact that with D = 150 meters, for example, ~t is about 1 microsecond. To obtain five significant figure precision, ~t must be measured to a precision of ± 1 X 10-11 second. Radiations and Telescopes / 235 • The most successful classical method of measuring c, notably by Michelson, employed a constant light source, a rotating multi-sided mirror as the effective time base, a distance D ~ 35 km, and a measurement of the consequent deflection of the focussed return beam that resulted from the rotation of the mirror during the lapse of time dt. • Modern indirect methods give the presently accepted value, c = 2.99792456(2) X 105 km s-l. 818. 499 seconds or 8.32 minutes. 819. 63,200 AU per light year. 820. The Earth travels 211" AU per year. Therefore, the speed of light is about 10,000 times the orbital speed of the Earth. 821. The intensity varies inversely as the square of the distance. 822. 1,600 times as great as at Pluto. 823. 900 times as great as at Neptune. 824. 27/1. 825. 27 meters diameter. 826. 55 meters diameter. 827. 1/16. 828. lOA times brighter than at the Earth. 236 / Radiations and Telescopes 829. The Pioneer 10 instruments are powered by four radioisotope terminal generators which derive electrical power from the decay products of plutonium 238. This electrical power is converted to heat. Also there are several smaller plutonium sources used for heaters at particular points in the spacecraft. Hence, the temperatures of all elements of the spacecraft are nearly independent of sunlight at its great distance from the Sun. 830. (a) The amount of sunlight that it absorbs. 831. About 2.7 degrees Kelvin. 832. 87 km s-1. 833. (c) Assumes that the speed of light is independent of the relative motion of the source and the observer. 834. (d) The laws...


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