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Homework Help: Need Help With Lens

  1. Apr 11, 2005 #1
    Need Help With Lens!!

    An object is placed 10cm in front of a convex lens of focal length 20cm. Where is the image formed and what is its magnification??..Your solution should include an accurately drawn ray diagram???

    Pleeez help me with this question???...im so stuck??

  2. jcsd
  3. Apr 11, 2005 #2
    Use the lens formula:

    1/f = 1/u + 1/v

    where f = focus, u = object distance from lens, and v = image distance from lens


    1/20 = 1/10 + 1/v

    Solve for v
    v = -20cm

    So the image is located at 20cm
    Last edited: Apr 11, 2005
  4. Apr 11, 2005 #3


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    You should have at least[/b] be able to know where to start! The lens equation would have been a good one. If you didn't even know this, then you should review the material again and know what that equation is for, and how to use it.

    If you do that, then you would have been able to derive what IntuitioN did, AND, be able to decipher what the "v = -20 cm" answer means (i.e. what does the NEGATIVE sign signifies). So look at the lens equation, and understand how each symbol are defined.

  5. Apr 11, 2005 #4
    oh right, thank you...so when i work out the magnification would it be 20/10 which is 2???
  6. Apr 11, 2005 #5


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    And for your ray diagram draw the lens with a compass. Then put in the object and draw the rays.
  7. Apr 11, 2005 #6
    the ray diagram should have given you a clue to the image's location, magnification, and orientation. it is actually pretty easy.
  8. Apr 11, 2005 #7
    magnification formula is [tex] m = - \frac{d_{i}}{d_{o}} [/tex]
    where di is the image distance and do is the object distance and the negative sign means that if it was positive then the image is in the same direction as the object, if neaitve then the opposite direction.
    thi DIRECTION means that if the object were an arrow poiting upward, then a negatie magniification would mean that the image arrow would point downwwward, signifying an inverted image
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