# Building a telescope

1. ### frostking

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

2. ### ideasrule

2,321
There's a formula that relates the magnification of a magnifying glass with its focal length. Do you know what it is?

3. ### frostking

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

4. ### nasu

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.

5. ### frostking

21
Thanks so much. I will remember this and use it! Frostking

6. ### ideasrule

2,321
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.