What is the focal point of a lens in a geometrical optics problem?

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Homework Help Overview

The discussion revolves around understanding the concept of the focal point of a lens within the context of geometric optics. Participants are exploring the implications of the problem statement regarding the focal length and the necessary considerations for solving it.

Discussion Character

  • Conceptual clarification, Assumption checking, Exploratory

Approaches and Questions Raised

  • The original poster attempts to clarify whether the focal point is correctly identified as -25.5 cm and questions the need for additional parameters like radii or refractive index. Some participants suggest a two-step approach to the problem, treating the lens surface as a mirror to deduce further information.

Discussion Status

Participants are actively engaging with the problem, with some providing hints and confirming the validity of approaches. There is a recognition of the need to explore the relationship between the lens surface and its focal length, indicating a productive direction in the discussion.

Contextual Notes

There is mention of treating the lens surface as a spherical mirror and using related equations, which suggests that assumptions about the lens's properties are being examined. The original poster appears to be uncertain about the requirements of the problem.

Marioweee
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Homework Statement
We have a divergent lens on which a beam of parallel rays strikes from left to right. Part of the light reflects off the first face and the reflected rays converge at the point indicated by the arrow, which is - 25.5 cm from the vertex of the first lens surface. Assuming that the lens is symmetrical, that is, that "R1= -R2 &, which is thin, and whose refractive index is 1.5, state the value of the focal point of the lens with its sign.
Relevant Equations
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I have recently started with geometric optics and I do not quite understand what this problem asks of me. According to the statement, the focal point of the lens would be -25.5cm, right? That is, it is only a problem of concepts where it is not necessary to take into account the radii of the lenses or the refractive index, right?
Maybe I am very wrong that is why I ask for some help.
This is the picture of the problem that I have done.
WhatsApp Image 2021-10-03 at 20.02.34.jpeg
 
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Marioweee said:
According to the statement, the focal point of the lens would be -25.5cm, right?
Why would you think that? Hint: Solve the problem in two steps. Step one: Treat the first surface of the lens as a mirror. What can you then deduce?
 
If the first surface is treated as a spherical mirror then f=R/2. From this equation we can determine the value of R1. Then, from the Lensmarker's equation I could determine the focal length of the system, right?
 
Marioweee said:
If the first surface is treated as a spherical mirror then f=R/2. From this equation we can determine the value of R1. Then, from the Lensmarker's equation I could determine the focal length of the system, right?
Right!
 
Thank you very much for your help
 

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