# Telescope Optics

Tags:
1. Jan 3, 2014

### chemphys1

1. The problem statement, all variables and given/known data

*The diagram of the telescope is attached*

Question states

A telescope is constructed from five thin lenses of Diameter D and focal lengths 2F, F, F,F,2F
located at x = 0,2F,4F,6F,8F
Trace the path of a ray entering the system parallel to the optical axis

3. The attempt at a solution

This is undergraduate lab in chemistry but I haven't done physics before, I understand the middle line is the optical axis, but not much more. I've tried to do some reading but nothing seems to relate back to this question as it has little information about the type of lense/telescope and I seem to just be confusing myself completely

Any help really appreciated, quite confused!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jan 3, 2014

### Vespa71

No tilts uninteresting

3. Jan 3, 2014

### chemphys1

Sorry I don't follow?

4. Jan 3, 2014

### Staff: Mentor

Disregard his reply. He is saying that the problem is fairly easy as stated, but it's an intro course so there is nothing wrong with starting with straightforward questions.

I'll post a reply to your OP in a sec...

5. Jan 3, 2014

### Staff: Mentor

One Relevant Equation that you should have posted is the "Lens Equation". Can you post it? And show a diagram demonstrating what it means for ray tracing through a single thin lens?

6. Jan 3, 2014

### chemphys1

Ok thanks

Lens equation for a thin lens is 1/f = 1/d1 + 1/d2 ?

where f is the focal length of lens, d1 is image to lens distance, d2 object to lens distance

Not too sure about the lens, how do I know if the lens is concave or convex in this question?

7. Jan 3, 2014

### Staff: Mentor

If the focul length is positive, it is convex (focuses a real image). Are you sure the problem statement is complete, and all of the focul lengths are positive as shown in your OP?

8. Jan 3, 2014

9. Jan 3, 2014

### chemphys1

Yes all the focal lengths are positive,
the only difference between the question and my diagram is the diagram has arrows pointing up and down on the lenses, does that indicate convex too?

10. Jan 3, 2014

### Staff: Mentor

I don't think so. As you can see in the Google Images, arrows are sometimes used to show when an image is inverted or not. Can you post a scan of the actual problem image?

11. Jan 3, 2014

### chemphys1

attached

#### Attached Files:

• ###### photo (20).JPG
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12. Jan 3, 2014

### Staff: Mentor

Oh, double arrows. Yes, that might also imply convex/converging lenses.

So can you draw the first ray, coming in from the left about half-way up the first lens? Where does it cross the centeral axis?

13. Jan 4, 2014

### chemphys1

Like that? crossing at focal point F after the lens?

#### Attached Files:

• ###### Untitled 2.png
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14. Jan 4, 2014

### chemphys1

is the whole pathway of the ray this? keeps going back through the next focal point?

#### Attached Files:

• ###### Untitled 3.png
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15. Jan 4, 2014

### Staff: Mentor

The first lens has a focul length of 2F, not F.

16. Jan 4, 2014

### chemphys1

How does that affect the ray, does it not converge to the focal point at 3f?

would it hit the optical axis at 2F on the x axis, through the centre of the 2nd mirror?

17. Jan 4, 2014

### chemphys1

this?

#### Attached Files:

• ###### Untitled 4.png
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18. Jan 4, 2014

### Malverin

This may help you. Here you can see the ray paths.

http://mcat-review.org/light-geometrical-optics.php

19. Jan 4, 2014

### chemphys1

Sorry thank you for all the help but I'm really not understanding

The question says trace a ray, will I have 1 line like I did in my diagram or 3 lines like these images?
I literally knew nothing on optics before this question and my google searches were getting me more confused, including all these diagrams with lines going everywhere, multiple focal points etc.

Is my attempted diagrams completely wrong?

20. Jan 5, 2014

### Malverin

That is your problem statement:

So you have to trace only the parallel ray, but you will need all 3 cases to draw it to the end.