Physics final exam problem: microscope object distance

Physics 205B Final Exam, spring semester 2017
Cuesta College, San Luis Obispo, CA

A NASA education guide for high school students[*] details plans for a simple microscope using an f = +28 mm objective lens and an f = +46 mm eyepiece lens, separated by a lens-to-lens distance of 160 mm. Determine the distance (in mm) that an object should be placed at in front of the objective lens for viewing through this microscope. Show your work and explain your reasoning by using ray tracings and/or thin lens equations, properties of lenses, images, and magnification.

[*] nasa.gov/pdf/350502main_Optics_Building_a_Microscope.pdf.

Solution and grading rubric:

• p:
  Correct. Determines the object distance d_o1 for the objective lens by discussing:
  1. that the lens-to-lens distance is equal to the objective image distance d_i1 plus the eyepiece focal length f₂ (as the image produced by the objective is placed at the focal point of the eyepiece), such that d_i1 = 160 mm − 46 mm = 114 mm; and
  2. knowing the focal point f₁ = +28 mm and the image distance d_i1 = 114 mm for the objective lens, uses the thin lens equation to find the object distance d_o1 for the objective lens.
• r:
  Nearly correct, but includes minor math errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least recognizes the placement of the intermediate image produced by the objective lens between the objective lens and eyepiece lens.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Garbled attempt at applying ray tracings and/or thin lens equations, properties of lenses, images, and magnification.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying ray tracings and/or thin lens equations, properties of lenses, images, and magnification.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.
  

Grading distribution:
Sections 30882, 30883
Exam code: finalmR3x
p: 1 student
r: 1 student
t: 5 students
v: 14 students
x: 3 students
y: 1 student
z: 1 student

A sample "p" response (from student 7117):