1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding lens diameter - German physics task

  1. Jul 9, 2015 #1
    Hey everybody,

    I'm right now trying to solve a question I found in a German physics competition 2006 concerning the diameter of a thin camera lens. I'm not able to find a solution, since there is basically nothing given.
    Here the task (I underlined phrases which might be important):

    A ruler was photographed. There was a chain of lights with small lamps paced far behind the ruler, so that the lamp occur to unclear. You are allowed to assume, that the picture was taken by a single, very thin lens.
    Find the diameter of the lens by using the photo.

    The only given value is the diameter of the unclear circles caused by the small lamps (by using the ruler on the photo). I know some laws and how to make these "ray diagrams" with lines, but I have no Idea how to solve this task and what is needed, especially since I have never seen a formula (or so) that includes the diameter.

    I hope you can help me finding a solution.
     

    Attached Files:

    • bild.png
      bild.png
      File size:
      120.5 KB
      Views:
      102
  2. jcsd
  3. Jul 9, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Can you show that?

    If the focal length is f, how far away from the ruler is the camera?
     
  4. Jul 9, 2015 #3
  5. Jul 10, 2015 #4

    Lok

    User Avatar

  6. Jul 10, 2015 #5

    Lok

    User Avatar

    Where would an object at exactly f focus?
     
  7. Jul 10, 2015 #6
    Well, the principle ray and the focal ray would occur to be nearly parallel, since they are so far away, and the rays would meet very close after passing the lens.
     
  8. Jul 10, 2015 #7

    Lok

    User Avatar

    So where would the far off lights focus? And what would happen with the rays afterwards?
     
  9. Jul 10, 2015 #8
    Mhh, maybe it is not even possible to project such an object on the other side, since the focal ray would have to point upwards.
     
  10. Jul 10, 2015 #9

    Lok

    User Avatar

    This just means that the ruler is not at f.
    And you can also establish where it is, "between lens and f" or from "f outwards".
     
  11. Jul 10, 2015 #10
    Since they are nearly parallel, they would focus (near to or) in the focal point. After meeting, the rays would diverge.
     
  12. Jul 10, 2015 #11
    That's already a nice aspect for me to understand the situation - thank you. But how will this knowledge help me to find the diameter?
     
  13. Jul 10, 2015 #12

    Lok

    User Avatar

    Now you will get two things on the same screen (there must be one).
    One image focused with a graded ruler and one unfocused diverged ray which overlays the ruler on the screen.
    You could get the answer by drawing only.
     
  14. Jul 10, 2015 #13
    yes. a nice problem. I haven't seen it before.
     
  15. Jul 10, 2015 #14
    I hope I got the situation. Sorry for my terrible drawing (as I noted on the picture, the rays coming of the lamp are supposed to be parallel).
     

    Attached Files:

  16. Jul 10, 2015 #15

    Lok

    User Avatar

    I agree with the adjective. :D
    You need a screen, which is also positioned somewhere between lens f and outwards.
    I insist on the drawing as the math is really easy to derive form it.
     
  17. Jul 10, 2015 #16
    Is it possible to place the screen on the position (behind the lens) where I wrote ruler?
     
  18. Jul 10, 2015 #17

    Lok

    User Avatar

    Oh, sorry for not noticing earlier.
    But you set the ruler between f and ens, where would it focus and what type of image would it produce?
     
  19. Jul 10, 2015 #18

    Lok

    User Avatar

    Please check the ruler's (real one) position as stated above.
    This would allow you to set a screen behind the lens in the appropriate place for a focused projection to take place.
     
  20. Jul 10, 2015 #19
    a virtual image?

    Second try! This time I improved my drawing. :D
     

    Attached Files:

  21. Jul 10, 2015 #20
    Lok is correct. You need a nice geometric drawing done with a ruler. Then it becomes a matter of comparing triangles. Other than that, I'm only getting in the way, here.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Finding lens diameter - German physics task
  1. Finding diameter (Replies: 1)

Loading...