Building a Telescope: Calculating Total Magnification & Length

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Homework Help Overview

The discussion revolves around building a telescope using two magnifying lenses with magnifications of 2x and 5x. The original poster seeks to determine the total magnification, identify which lens should serve as the objective, and calculate the length of the telescope.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster attempts to calculate total magnification and determine the appropriate lens configuration but expresses uncertainty about calculating the telescope's length due to missing focal length information. Some participants question the necessity of specific formulas for telescopes versus magnifying glasses.

Discussion Status

Participants are exploring the relationship between magnification and focal lengths, with some guidance provided regarding the formulas applicable to telescopes. There is an acknowledgment of the original poster's confusion regarding the required information for calculations.

Contextual Notes

There is mention of constraints related to exam rules on formula usage, as well as the original poster's uncertainty about the focal lengths of the lenses being used.

frostking
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Homework Statement


I need to build a telescope from two magnifying lens, one is 2x the other is 5x. First I need to give the total magnification possible, state which lens to use as the objective and finally to state the length of the telescope.


Homework Equations


l Ml = Fo/Fe total magnification is lM1l x lM2l Length = Focal length of objective + focal length of eyepiece


The Attempt at a Solution

The total possible magnification is 10X. I use the 2X for the objective because you need a long focal length and smaller magnification. I am stuck on how to get the length of the scope. I know the absolute value of M and can say that Fobjective is equal to lMl times F of eyepiece. But I don't have either focal length as far as I can tell and so I have no way to determine the length. I have the answer, so I know it should be 55 cm but why? Any help would be much appreciated. Frostking
 
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There's a formula that relates the magnification of a magnifying glass with its focal length. Do you know what it is?
 
Yes, silly me it is M = 25/F On exams we are only allowed to use formulas on our formula sheet or that can be derived from the formula sheet. This formula is NOT provided. I guess I will just have to memorize this and hope that it is allowed to be used on the final. Thanks and sorry for wasting your time. Frostking
 
This formula is valid for magnifying glass. You don't need it for telescope.
For a telescope the magnification is given by the ratio of the two focal lengths.
 
nasu said:
This formula is valid for magnifying glass. You don't need it for telescope.
For a telescope the magnification is given by the ratio of the two focal lengths.

Thanks so much. I will remember this and use it! Frostking
 
You don't need the formula for a telescope, but the specifications of the magnifying glasses are given. I can't see how you would find the ratio of the focal lengths without using these specifications.
 

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