Two lense system - Homemade spyglass.

In summary: The distance between the lenses controls the "seeing angle". The larger the distance, the smaller the angle. If you were to hold the spyglass up to your eye and look at an object close by, you would need to adjust the distance between the lenses in order to see the object clearly. However, if you were to look at an object far away, you would only need to adjust the distance between the lenses if you wanted to see the object in its entirety.
  • #1
Casper^^
3
0
Hello.

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 length 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. Homework 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)

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 that's the only connection i can find.

I guess, what i am basicly asking is: What exactly happens inside the spyglass when I adjust the length 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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  • #2
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
 
  • #3
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 :)
 
  • #4
Assuming they are simple lenses (not achromats) you can easily work out the radius of curvature form the focal length - but that's 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'
 
  • #5
Right, thank you.
I know that i can calculate the radius with the lensemakers formula, 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?
 

FAQ: Two lense system - Homemade spyglass.

How do I make a homemade spyglass using a two-lens system?

Making a homemade spyglass using a two-lens system is relatively simple. You will need two lenses, such as magnifying lenses or eyeglass lenses, and a tube or cardboard cutout to hold them in place. Place one lens at each end of the tube and secure them in place. Your spyglass is now ready to use!

What are the benefits of using a two-lens system for a homemade spyglass?

A two-lens system allows for better magnification and clearer image quality compared to using a single lens. This is because the two lenses work together to focus the light and produce a sharper image. Additionally, using two lenses also allows for a longer focal length, making it easier to spy on objects from a distance.

What materials can I use for the tube or cardboard cutout in a homemade spyglass?

You can use a variety of materials for the tube or cardboard cutout in a homemade spyglass. Some common options include toilet paper rolls, paper towel rolls, PVC pipes, or even rolled-up paper. The important thing is to make sure the material is sturdy enough to hold the lenses in place and is long enough to achieve the desired focal length.

Can I adjust the focus on a homemade spyglass with a two-lens system?

Yes, you can adjust the focus on a homemade spyglass with a two-lens system. This can be done by sliding the lenses closer or further apart within the tube or by moving the tube closer or further away from the object you are spying on. Experiment with different adjustments until you find the best focus for your spyglass.

Are there any safety precautions I should take when using a homemade spyglass with a two-lens system?

Yes, there are a few safety precautions to keep in mind when using a homemade spyglass. First, avoid looking directly at the sun or other bright light sources through the spyglass, as this can damage your eyes. Additionally, be careful when handling the lenses to avoid scratching or breaking them. And finally, always make sure to supervise children when using the spyglass to prevent any accidents.

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