## 20150624

### Physics final exam question: converging lens image same size as object

Physics 205B Final Exam, spring semester 2015
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 23.63

A Physics 205B student places a object 22 cm in front of (to the left of) a converging lens (f = +11 cm). Discuss why the size of resulting image will be exactly the same size as the original object. Explain your reasoning by using the properties of lenses, thin lens equations and/or ray tracings.

• p:
Correct. Image will be the same size as the original object because:
1. from the thin lens equation, the image distance di will be the same as the object image distance do;
2. and from the linear magnification equation, the image height h' will be –hdi/do, which makes the image the same size as (but inverted from) the original object.
• r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically shows that do = di, but does not explicitly relate this to how the image and object sizes compare.
• t:
Nearly correct, but argument has conceptual errors, or is incomplete.
• 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:
Blank.
Sections 30882, 30883
Exam code: finalLd0c
p: 16 students
r: 18 students
t: 2 students
v: 2 students
x: 1 student
y: 1 student
z: 3 student

A sample "p" response (from student 5425):

A sample "r" response (from student 1675):