Physics final exam problem: reconstruction of Galileo's telescope

Physics 205B Final Exam, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 24.49, 24.51, Comprehensive Problem 24.63

The Museo Galileo Institute and Museum of the History of Science in Florence, Italy has several of Galileo's telescopes in its collection[*]:

Original telescope [late 1609-early 1610] made by Galileo consisting of a main [adjustable length] tube with separate housings at either end for the objective and the eyepiece... The objective [has] a focal length of 980 mm... The original eyepiece was lost. The instrument's magnification is [–]21×.

Assume that the lost eyepiece was a converging lens. Determine (a) the focal length of the eyepiece and (b) the length of the telescope (as measured from the objective to the eyepiece) when used to observe distant objects. Show your work and explain your reasoning.

[*] brunelleschi.imss.fi.it/museum/esim.asp?c=405002.

Solution and grading rubric:

• p:
  Correct. Determines (1) the focal length of the eyepiece, and (2) the length of the telescope.
• r:
  Nearly correct, but includes minor math errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least has focal length of eyepiece, and some attempt in finding the length of the telescope.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. At least attempts to use telescope equations.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Use of microscope equations, etc.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882
Exam code: finalpL3x
p: 17 students
r: 2 students
t: 7 students
v: 0 students
x: 2 students
y: 1 student
z: 1 student

A sample "p" response (from student 1224):
A sample "y" response (from student 1101), invoking lyrics from Queen's "Bohemian Rhapsody":