Physics midterm problem: microscope construction

Physics 205B Midterm 1, spring semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 24.42

The converging lenses available from a commercial optics supplier have focal lengths f = +0.30 cm and f = +0.45 cm, respectively[*]. A Physics 205B student would like to use these two lenses to construct a microscope with a "tube length" L (the distance from focal point to focal point) of 5.0 cm, where the f = +0.30 cm lens is used as the objective. Solve for (a) the angular magnification of this microscope, and (b) the distance from the object to the objective lens. (The near point of the Physics 205B student is 25.0 cm.) Show your work and explain your reasoning.

[*] edmundoptics.com/optics/optical-lenses/double-convex-dcx-spherical-singlet-lenses/uncoated-double-convex-dcx-lenses/1748.

Solution and grading rubric:

• p:
  Correct. Determines (a) angular magnification to be –930×, and (b) the object must be placed 0.32 cm in front of the objective lens.
• r:
  Nearly correct, but includes minor math errors. Determines angular magnification, but does not explicitly solve for the distance for the object in front of the objective lens, but instead understands that it must be held very near outside the focal point (0.30 cm) of the objective lens.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least has magnification, and some attempt in finding the distance for the object in front of the objective lens
• :
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner.
• x:
  Implementation of ideas, but credit given for effort rather than merit.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 30882
Exam code: midterm01p0C4
p: 19 students
r: 2 students
t: 4 students
v: 6 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1224):