Question regarding a two-lens system

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Homework Help Overview

The discussion revolves around a two-lens optical system involving two converging lenses, each with a focal length of 15 cm, positioned 20 cm apart. An object is placed 30 cm in front of the first lens, leading to questions about the location of the final image and the system's magnification.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the image distance calculated as 30 cm and express confusion regarding the spatial arrangement of the lenses. There is mention of using a ray diagram to visualize the paths of light rays and how they interact with the lenses. Some suggest treating the image formed by the first lens as the object for the second lens, while others raise questions about the sign conventions for object distances.

Discussion Status

The conversation is ongoing, with participants exploring different interpretations of the image formation process. Some guidance has been offered regarding the treatment of the first image as the object for the second lens, but no consensus has been reached on the final image location or magnification.

Contextual Notes

There is a noted complexity due to the proximity of the lenses and the distances involved, which may lead to confusion about the image formation process. Participants are also considering the implications of sign conventions in their calculations.

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Homework Statement


Two converging lenses, each of focal length 15 cm, are placed 20 cm apart, and an object is placed 30 cm in front of the first. Where is the final image formed and what is the magnification of the system?

Homework Equations


Mtot = M1 * M2
(1/f) = 1/do + 1/di

The Attempt at a Solution


[/B]
Okay, I was able to find the image distance would be 30 cm, but I have no idea how that would happen if the lenses are 20 cm apart. When drawing the ray diagram, the rays touch the second lens before converging, so I don't know where to go from there. Any help would be appreciated. Thanks!
 
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pisquared said:

Homework Statement


Two converging lenses, each of focal length 15 cm, are placed 20 cm apart, and an object is placed 30 cm in front of the first. Where is the final image formed and what is the magnification of the system?

Homework Equations


Mtot = M1 * M2
(1/f) = 1/do + 1/di

The Attempt at a Solution


[/B]
Okay, I was able to find the image distance would be 30 cm, but I have no idea how that would happen if the lenses are 20 cm apart. When drawing the ray diagram, the rays touch the second lens before converging, so I don't know where to go from there. Any help would be appreciated. Thanks!
One way is to take the image that would be formed by the first lens and treat it as the source object for the second lens. You just have to be careful about which side the final image will be.
 
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pisquared said:
Okay, I was able to find the image distance would be 30 cm, but I have no idea how that would happen if the lenses are 20 cm apart. When drawing the ray diagram, the rays touch the second lens before converging, so I don't know where to go from there.
You can take that image as object of the second lens. As it is behind the second lens, take the object distance negative.
 
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@ehild @haruspex So the first image would go straight through the second lens and be 10 cm behind it, then be the object of the second lens?
 
pisquared said:
@ehild @haruspex So the first image would go straight through the second lens and be 10 cm behind it, then be the object of the second lens?
Yes.
 

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