Building a telescope

frostking

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

ideasrule

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

frostking

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

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

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

ideasrule

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.