One beam of coherent light travels path P1 in arriving at point Q and another coherent beam travels path P2 in arriving at the same point. If these two beams are to interfere destructively, the path difference P1 - P2 must be equal to
A)
an odd number of half-wavelengths.
B)
an even number of half-wavelengths.
C)
zero.
D)
a whole number of wavelengths.
E)
a whole number of half-wavelengths.
A
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Indicate whether the statement is true or false
A 30-year-old astronaut goes off on a long term mission in a spacecraft that travels at speeds close to that of light. The mission lasts exactly 20 years as measured on Earth. Biologically speaking, the astronaut's age would be
A) more than 50 years. B) exactly 50 years. C) less than 50 years. D) exactly 30 years. E) exactly 25 years.
Equilibrium: In the figure, a 10-lb weight is suspended from two spring scales, each of which has negligible weight. Which one of the following statements about the readings in the scales is true?
A. Each scale will read 5 lb. B. The top scale will read zero, the lower scale will read 10 lb. C. The lower scale will read zero, the top scale will read 10 lb. D. Each scale will show a reading between one and 10 lb, such that the sum of the two is 10 lb. However, exact readings cannot be determined without more information. E. None of these is true.
Volcanic activity on Venus is thought to be:
A) more frequent, but less violent than volcanic activity on Earth. B) more frequent and more violent than volcanic activity on Earth. C) less frequent, but more violent than volcanic activity on Earth. D) less frequent and less violent than volcanic activity on Earth. E) the same as volcanic activity on Earth.