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Building a telescope

by frostking
Tags: building, telescope
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frostking
#1
Dec3-09, 07:48 PM
P: 21
1. The problem statement, all variables and given/known data
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.


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


3. 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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ideasrule
#2
Dec3-09, 09:57 PM
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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?
frostking
#3
Dec4-09, 12:41 AM
P: 21
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

nasu
#4
Dec4-09, 09:34 AM
P: 1,969
Building a telescope

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.
frostking
#5
Dec4-09, 09:46 AM
P: 21
Quote Quote by nasu View Post
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
ideasrule
#6
Dec4-09, 01:48 PM
HW Helper
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P: 2,323
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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