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The 300-m-diameter Arecibo radio telescope pictured in Figure 27.28 detects radio waves with a 4.00 cm average wavelength. (a) What is the angle between two just-resolvable point sources for this telescope? (b) How close together could these point sources be at the 2 million light year distance of the Andromeda galaxy?
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1. $1.63 \times 10^{-4} \textrm{ rad}$
2. $325 \textrm{ ly}$
Solution Video

# OpenStax College Physics for AP® Courses Solution, Chapter 27, Problem 57 (Problems & Exercises) (2:02)

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This is College Physics Answers with Shaun Dychko. The RAC Bow radio telescope has a diameter of 300 meters and it receives wavelengths of four centimetres. These are radio waves. And the question is what is the minimum resolvable angle between two objects for this type of telescope. So the really good criterion tells us that, that's 1.22 times lambda over d. So the 1.22 times four times ten to the minus two meters. That's the wavelength written in meters and divide that by the size of the mirror which is 300 meters. So this thing is as a mirror that reflects the radio waves up to a sensor that's not shown in the picture it would be somewhere up here. So that's 1.63 times ten to the minus four radians is the minimum angle between two objects in order to distinguish them. And now part b says consider these objects to be two million light years away, how far are they separated given this minimum angle. Well we can think of this as a triangle between the two objects here and here and this angle is theta that we just found in part a. And so tangent of theta is gonna be this delta Y distance between the objects divided by the adjacent which is the distance from the earth here to one of the objects. So solving for delta Y by multiplying both sides by x we get the delta Y is x Tan theta. So that's two times ten to the six light years times tangent of 1.6267 times ten to the minus four radians that we figured out up here. And that gives a 345 light years is the smallest separation that we can discern with this radio telescope.