Optics Question, where is the lens

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Homework Help Overview

The discussion revolves around an optics problem involving a lens, where a 3 cm tall object is placed 11 m from a wall, and the goal is to project an upside-down image using a lens with a 20 cm focal length. Participants are exploring the type of lens, its placement, the height of the image, and magnification.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculations related to the lens placement and magnification, with one participant questioning the clarity of the measurements used. There is a suggestion to sketch a ray diagram to verify the results, and another participant emphasizes the importance of specifying the reference point for distance measurements.

Discussion Status

Some participants have provided feedback on the calculations and the need for clarity in the answers. There is an ongoing exploration of the assumptions made in the problem setup, and participants are encouraged to refine their reasoning based on the physical principles of optics.

Contextual Notes

Participants note that the textbook does not provide answers, leading to uncertainty about the correctness of the calculations. There is also a mention of the need for units in the results, and the implications of the focal length on the expected distances and magnification are being questioned.

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Homework Statement




A 3 cm tall object is placed 11 m from a wall. If the image is to be projected upside down with a 20 cm focal lense, what is:

A} The type of lens used?

B] Where is the lens placed?

C] What is the height of the image?

D] What is the magnification of the lens?




Homework Equations




1/f = 1/d_i + 1/d_o


The Attempt at a Solution



I have gotten the answers:

B] 0.2037
C] -1.5899
D] -69.999

My textbook does not supply answers, therefore I am unsure if mine are correct.

Thanks for the help,
OPO
 
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oneplusone said:

Homework Statement




A 3 cm tall object is placed 11 m from a wall. If the image is to be projected upside down with a 20 cm focal lense, what is:

A} The type of lens used?

B] Where is the lens placed?

C] What is the height of the image?

D] What is the magnification of the lens?




Homework Equations




1/f = 1/d_i + 1/d_o


The Attempt at a Solution



I have gotten the answers:

B] 0.2037
C] -1.5899
D] -69.999

My textbook does not supply answers, therefore I am unsure if mine are correct.

Thanks for the help,
OPO
Pls post your working.
 
The way to check your working in optics is to sketch out the ray diagram and see if it appears to agree.
You can also check be referencing related physics - i.e. how would you find the focal length of a lens using a very distant object? Therefore, how far should the lens be from the screen in your example (roughly)? Should it be farther or closer than that?

Note:
- the lensmakers formula usually sits the lens at x=0, and other measurements relative to this. It is not clear where your results are measured from or how you have used the formula.
- your results are meaningless without units.
 
Thanks, I have added units.
I sketched out a diagram, and ended up lettingd_i = 11 - d_odue to the problem restrictions.
oneplusone said:

Homework Statement

A 3 cm tall object is placed 11 m from a wall. If the image is to be projected upside down with a 20 cm focal lense, what is:

A} The type of lens used?

B] Where is the lens placed?

C] What is the height of the image?

D] What is the magnification of the lens?

Homework Equations

1/f = 1/d_i + 1/d_o

The Attempt at a Solution



I have gotten the answers:

B] 0.2037 m
C] -1.5899 m
D] -69.999 X

My textbook does not supply answers, therefore I am unsure if mine are correct.

Thanks for the help,
OPO
 
And your working?
 
... and where you are taking these distances from?
i.e. you wrote "B: 0.2037m" ... that does not actually answer the question does it?
If you asked, "where is the toilet?" and I said "6m" - how would you react?

The answer should look like: "The lens should be placed 20.4cm from ..." where? The screen? The object?

... and what about the other questions I asked?
... did you use your sketch to see if the answers you got made sense?
i.e. that 20.4cm is very close to the focal length. Does that make sense?
given that distance - does it make sense for the magnification to be so big?
 
Last edited:

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