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Homework Help: Experiment: Measuring the focal length of a converging lens

  1. Nov 28, 2016 #1
    1. The problem statement, all variables and given/known data
    You are required to set up a light source, an object grid, a converging lens and a screen so that a quantitative investigation of object and image distances can be performed. Tabulate corresponding object and image distances (p and p' respectively). Also tabulate values of 1/p, 1/p', p+p' and pp'.

    2. Relevant equations
    Theory suggests that 1/p + 1/p' = 1/f. Graph (i) p' versus p, (ii) 1/p' versus 1/p, (iii) pp' versus p+p'. Explain the shape of each graph. Determine its value in each case. Which method do you think its most accurate?

    State, with reasons, the level of accuracy expected from each of your measurements and hence give a statement of how accurate you expect from each of your measurements and hence give a statement of how accurate you expect your value of f to be in each case.

    3. The attempt at a solution

    I set up the experiment as shown in the pictures. I placed the object grid in front of the light source, followed by a plano convex lens (f=30 cm) followed by the screen. The problem with the object grid, I don't see an image of the object grid but of an inverted filament instead (not clear in the diagram). I then removed the object grid and I still saw an inverted image of the filament. Why am I not seeing the object grid? Everything is aligned. I only see the object grid very clearly when it's really close to the source after that it disappears and a very clear image of the filament appears.

    Another issue is that when I measure the object distance on the meter ruler and and subtract it from the image distance which I also read from the meter ruler, I don't get the focal length actually mentioned on the lens pack. The focal length on the pack is 30 cm and I get a difference of 85 +/- 5.

    Attached Files:

  2. jcsd
  3. Nov 28, 2016 #2
    Try placing the object grid as close to the light source as you can; the grid is your object, and the source creates the rays you are trying to direct into your lens.

    Hope this helps!

    - Strelkov
  4. Nov 28, 2016 #3
    Hi Strelkov,

    Thanks for your reply. I tried doing that too, as I moved the lens further away, the object grid would turn into this blur circle of light and then suddenly, after further movement, a very clear, inverted image of the filament would show up. Quite strange.
  5. Nov 28, 2016 #4

    Since you only have one lens, the resulting image will be inverted(recall ray trace diagrams). Once the image is clear and inverted, you have found the image; now use the lensmaker's formula to confirm your focal length.

    - Strelkov
  6. Nov 28, 2016 #5
    Yes I am aware that it will be inverted but my object is the grid and not the filament of the light source? And I don't get the focal length to be the same as what it said on its packaging?
  7. Nov 29, 2016 #6


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    Science Advisor

    Try this:
    Position the lens where you get
    Now move the light farther away from the lens and put the grid where the light used to be.
  8. Nov 29, 2016 #7
    Ok I'll try that but why can't I seem to get it with the light source present?

    I've attached a video of what I get. It's so clear.

    Can't seem to upload it :/
    Last edited: Nov 29, 2016
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