Physics midterm question: object moved away from converging lens

Physics 205B Midterm 1, spring semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 23.69

Placing an object to the left of a converging lens produces an inverted image. If the object is moved slightly farther from the lens, describe what will happen to the size of this image. Explain your reasoning by using the properties of lenses, thin lens equations and/or ray tracings.

Solution and grading rubric:
  • p:
    Correct. Uses ray diagram or thin lens equation(s) to explain why resulting real image will be smaller when object (already outside of the primary focal point) is moved away from the converging lens.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May argue as p increases, m = h'/h = –di/do decreases, but does not explicitly show that q decreases as well.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Explicit use of comparative ray tracings, or thin lens equations, but instead concludes image would increase in size as object distance is increased.
  • 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:

Grading distribution:
Section 30882
Exam code: midterm01o1Ls
p: 16 students
r: 3 students
t: 5 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 0524), using the thin lens equations:
A sample "p" response (from student 1123), using both ray tracings and thin lens equations:
A sample "t" response (from student 1227), using a ray tracing to show that the image size would increase:

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