Physics final exam question: telescope construction

Physics 205B Final Exam, fall semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 24.49

A Physics 205B student has a set of converging lenses with focal lengths +0.50 cm, +6.0 cm, +70.0 cm, and +150.0 cm. Describe the two lenses that should be selected as the objective and the eyepiece to make a telescope with the largest magnifying power, and what the distance between the objective and eyepiece lenses should be. Explain your answer using equations and/or diagrams.

Solution and grading rubric:

• p:
  Correct. Chooses longest focal length lens for objective, and shortest focal length lens for eyepiece, and the distance between the objective and eyepiece is the sum of their focal lengths. May clearly show how the objective primary focal point and eyepiece secondary focal point coincide, in lieu of stating L = f_o + f_e.
  
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
  
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
  
• x:
  Implementation/application of ideas, but credit given for effort rather than merit.
  
• y:
  Irrelevant discussion/effectively blank.
  
• z:
  Blank.
  

Grading distribution:
Section 70856
p: 8 students
r: 0 students
t: 1 student
v: 0 students
x: 0 students
y: 0 students
z: 1 student

A sample "p" response (from student 8110):