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Homework Help: Lens Laboratory

  1. Apr 2, 2007 #1
    I need a little help with a laboratory class I had about lens.

    We had a light source with a draw of an arrow on it, this light go through a lens of +100 mm (focal length) and a surface on the other side of the lens were we can see the arrow on it.
    We put the light source at 50cm from the lens (distance s) and we place the surface at a distance were the image looked good, and measure the distance from the lens and the screen and called p. Then we moved the light source 5 cm closer to the lens and adjust the screen again.

    We do this for about 10 times then we made a graphic 1/p VS 1/s and found a line on the graphic with m=-0.764 and b=0.0883.

    Later on, in the laboratory manual is asking for the meaning of b (the y-intercept).

    My question is: What is b? I actually read the physics book and I didn't found anything.
  2. jcsd
  3. Apr 2, 2007 #2
    Take a look at the lens maker equation?
  4. Apr 2, 2007 #3
    I done it already

    My equation would be 1/p=m * 1/s + b
    Lens maker equation is 1/f = (n-1) (1/r-1/R)

    I suppose that 1/s is (1/r-1/R) and that m is (n-1) but b ...
  5. Apr 2, 2007 #4
    Lets see if this is right

    (n-1) * (1/r-1/R) = m*1/s + b

    Ok since I got a negative m in a converging lens I can suppose that
    (m* 1/s) = -1*(1/r-1/R)

    and that

    n*(1/r-1/R)= b

    But b is a quality of the lens, Does have a relation with the focal length?

    PS. Sorry if I'm giving you hard time with my english, I'm trying,I'm trying...
  6. Apr 2, 2007 #5


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    Gold Member

    Supposing that 1/s is 1/r and 1/p is 1/R gives a different interpretation.

    In this case b = 1/(f*(n-1)), which has the correct dimension L^-1
  7. Apr 5, 2007 #6
    I found the equation that describe the object and image distance with the focal length on the physics book, it was simple.

    I think I got a lot of error on the data because I was supposed to had m=-1 then I would get:

    1/p = -1/s + b ;b=1/f

    then getting the equation 1/f=1/p+1/s

    Thanks for the help
  8. Apr 5, 2007 #7


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    Gold Member

    Good show. I was worried by the sign of your slope. Maybe you should make one of the distances negative ?
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