Physics final exam problem: smartphone projector

Physics 205B Final Exam, spring semester 2016
Cuesta College, San Luis Obispo, CA

An inexpensive magnifying glass mounted in a shoebox is used with a smartphone as a light source to project images on a wall in a darkened room.[*] Shown at right is a view of the enlarged, upright image of the smartphone inside the projector when looking through the magnifying glass. Discuss what adjustments must be made (if any) to the location and/or orientation of the smartphone in order to properly display an enlarged, upright image on a wall. Show your work and explain your reasoning by using the properties of lenses, thin lens equations and/or ray tracings.

[*] content.photojojo.com/diy/turn-your-phone-into-a-photo-projector-for-1.

Solution and grading rubric:
  • p:
    Correct. Since the smartphone's upright, enlarged virtual image cannot be projected onto a wall, and in order to produce an "upright" image projected on a wall, the smartphone must instead be:
    1. turned upside-down; and
    2. pushed further away from the magnifying glass (such that it is placed just outside the focal point).
  • r:
    Nearly correct, but includes minor math errors. Neglects to mention that the phone must be turned upside-down.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. May claim that the smartphone already projects an image onto a wall, or that only the magnifying glass to wall distance needs to be changed.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. No clear attempt at applying properties of lenses, thin lens equations, and ray tracings.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
Grading distribution:
Sections 30882, 30883
Exam code: finalm4u1
p: 6 students
r: 2 students
t: 4 students
v: 16 students
x: 9 students
y: 2 students
z: 0 student

A sample "p" response (from student 0001):

Another sample "p" response (from student 1614):

A sample "x" response (from student 1959):

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