For a plane wave approaching an obstacle with a rectangular opening, diffraction becomes important in which of the following circumstances?
A. the beam is coherent
B. one or more dimensions of the rectangular opening is near the wavelength of the source
C. both dimensions of the rectangular opening are larger than the wavelength of the source
D. only if the opening is circular
B. one or more dimensions of the rectangular opening is near the wavelength of the source
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A small car pushes on a disabled truck, accelerating it slowly forward. Each exerts a force on the other as a result of their contact interaction. Which vehicle exerts the greater force on the other?
A. The car B. The truck C. Both forces have the same magnitude. D. The truck doesn’t exert any force on the car. E. One needs more information to answer.
Your hand is warmed when you blow on it with your mouth open, but when your lips are puckered so that air expands as you blow, your hand is
A) warmer still. B) cooler. C) neither warmer nor cooler.
When does a protostar become a true star?
A) when the star is 1 million years old B) when the central temperature reaches 1 million Kelvin C) when nuclear fusion begins in the core D) when the thermal energy becomes trapped in the center E) when the stellar winds and jets blow away the surrounding material
Assume a gram of a substance marketed as "Pure Energy" is annihilated by a gram of a second substance "Anti-Pure Energy." How long would the energy released power a city which uses 109 watts of power?
A. 40 hrs B. 25 hrs C. 50 hrs D. 65 hrs E. 50000 hrs