Two lense system - Homemade spyglass.


This is my first time posting here, so bare with me. I am from Denmark and I am at my final year of what corresponds to Senior High School. I am currently working on a project about "optical instruments" in physics (I was told that our A level corresponded to your applied physics level). I'ts the biggest project we do the entire year, so I am a little stressed about it. Anyway, sorry for the introduction.

My question is regarding the path of light and the forming of a picture in a dobble lense system. Part of my assignment was to build my own spyglass. So the system in question consist of: One objective lense with a focal lenght of 30 cm and an ocular lense with a focal lengt of 5 cm. Both lenses have the same size. I know that, when looking at an object through the spyglass i have to ajust the objective lense in order to get a sharp image. But it is unclear to me, what exactlay happens with the lights path when i ajust the distance between the objective- and ocular lenses. I mean, in order to get a sharp image of an object at different distances i have to just the spyglass, but why is that?

2. Relevant equations
The magnification of the spyglass is given by f1/f2 = 30/5 = 6x times.
(Not really sure if any equations is needed for me question)

3. The attempt at a solution
Not sure. Maybe it have something to do with the fact that, the longer the object is away to smaller the viewing angle gets. Atlest thats the only connection i can find.

I guess, what i am basicly asking is: What exactly happens inside the spyglass when I adjust the lenght between the lenses in order to get a sharp image?

Sorry for the long post, i hope you can bare with me. Also i appologize now, if this forum is not intendeed for these sort of questions. Feel free to ask anything, I can provide more information if needed. Thank you for taking your time.
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Science Advisor
Homework Helper
Welcome to PF!

There is a slight difference depending if you have a 30cm convex and a 5cm concave (as Galileo) or two convex (as Kepler)
Either way it's easy to trace the rays with high school level optics (wiki geometrical optics)

For an object at a very large distance (like a planet) then there is a simple correct location of the lenses which gives you a parallel beams out, that is an image at infinity for an object at infinity
Wauv, thanks for quick answer and the welcome ;)

Sorry about that - Both lenses are double-convex. (the radius of the sphere is not known)
About the ray tracing - My question was more reagarding the theoretical explanation of the lights path through my spyglass. Actualy my assignment is to "1) Build a spyglass + 2) describe the way it work + 3) find its magnification".

So I have done 1 and 3 (only theoreticaly tho). But, like I said, I am unsure what happens* when i adjust the spyglass in order to get a sharp i mage.

*Sorry for the vague describtion. I mean what changes in the path of light, when i increase or decrease the distance between the two lenses, that provides the "sharp" image.

I hope this cleared things up a bid :)


Science Advisor
Homework Helper
Assuming they are simple lenses (not achromats) you can easily work out the radius of curvature form the focal length - but thats not important.

Tracing the ideal path through the system is easy - the lenses are just placed the sum of their focal lengths apart. But with spherical lenses in the real world there might be a position near this where some errors in the lenses cancel out. Without a computer ray tracing package and exact details of the lenses all you can do is move them and have some criterion for 'in focus'
Right, thank you.
I know that i can calculate the radius with the lensemakers formular, but like you said - not important :p

But what is it that provides the sharp image? Why is it that when i look at on object close by (2 m) i need to increase the distance between the lenses (more than 35 cm) and when looking at object far way (20 m) i decrease the distance (less than 35 cm).

When describing the functioning of the spyglass theorediacly, how should i describe that?

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