Why do large reflecting telescopes, like the Keck Observatories, have segmented mirrors?
What will be an ideal response?
Segmented mirrors of large telescopes reduce the weight of the mirror, therefore reducing the weight of the rest of the telescope, making it stronger and less expensive. Also, segmented mirrors prevents sagging (and thus breakage) of one, large, heavy mirror.
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Iron has an FCC structure at temperatures just above 912°C, and the lattice parameter is 0.3589 nm. Assume that BCC iron’s atomic radius is equal to 0.1241 nm as given in Appendix C, and for carbon it is 0.0077 nm. The atomic radius of iron should not increase with temperature and with changes in crystal structure; only the size of the lattice changes.
(a) Calculate the radius ratio of carbon to iron to see if it is appropriate for octahedral coordination. (b) Calculate diameter of the space available for the octahedral site at the center of the FCC unit cell, and compare this to the diameter of a carbon atom. (c) Comment on the diameter available relative to the diameter of the carbon atom and upon the possibility of solubility of carbon in FCC iron at octahedral interstitial sites. Also comment on the expected relative solubility of carbon in octahedral sites in FCC and BCC iron (see Problem 3.3).
How do we specify the location of a star on the celestial sphere?
A magnet on a frictionless mount would have its magnetic north pole point toward the Earth’s geographic
A. south pole B. north pole
Point charges of equal magnitudes (25 nC) and opposite signs are placed on (diagonally) opposite corners of a 60-cm × 80-cm rectangle. If point A is the corner of this rectangle nearest the positive charge and point B is located at the intersection of the diagonals of the rectangle, determine the potential difference, VB ? VA
a. ?47 V b. +94 V c. zero d. ?94 V e. +47 V