What is "stimulated emission"?
a. The interaction of a photon with an excited atom to cause it to emit another photon of the same wavelength.
b. Emission in response to an electric discharge through a gas instead of just random light emission.
c. Any excitation of atoms that later results in photons being given off by a substance.
d. Stimulation of the random action of electrons and photons creating scattered emission of light.
e. Emission of photons without requiring electron shell jumps.
a
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Which of the following methods used to determine the mass of a cluster of galaxies does not depend on Newton's law of gravity?
A) measuring the amount of distortion caused by a gravitational lens B) measuring the temperature of X-ray gas in the intracluster medium C) measuring the orbital velocities of galaxies in the cluster D) none of the above
Two bodies with masses m1 and m2 are both moving east with velocities of magnitudes v1 and v2, where v1 is less than v2 . The magnitude of the velocity of the center of mass of this system of two bodies is
a. less than v1. b. equal to v1. c. equal to the average of v1 and v2. d. greater than v1 and less than v2. e. greater than v2.
How do we determine the amount of dark matter in elliptical galaxies?
A) We measure the broadening of the galaxy's absorption lines to infer the speeds of stars at different distances from the galactic center. B) We measure how fast the galaxy rotates as a whole. C) We measure the orbital velocities of star-forming gas clouds in the outer portion of the galaxy. D) We count the number of stars in the galaxy. Combining this with the galaxy's volume, we can calculate the galaxy's density. E) We search for dark lanes of dust and black holes within the galaxy.
Which of the following statements best describes the limitations of Newtonian physics?
A) At very high speeds, Newtonian physics gives very incorrect predictions, and is correct in other situations. B) At very high speeds, or at very small sizes, or for gases and liquids, Newtonian physics gives very incorrect predictions. C) Newtonian physics is approximately correct, to within 1 or 2 percent, in all situations. D) At high speeds, or at small sizes, or for large distances, Newtonian physics is incorrect. E) As far as we know, Newtonian physics is correct in all situations.