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Homework Help: Optics of the magnifying glass

  1. Jun 19, 2016 #1
    1. The problem statement, all variables and given/known data
    An observer looks at a small object using a magnifying glass with the focal length of 12cm. Distance from his eye to the magnifying glass is 1cm. The shortest distance at which he sees clearly is 25cm and the largest is when the image of the object is in the distance infinite. Minimal and maximal values of the distance of the object from the magnifying glass at which the object is seen clearly is...
    2. Relevant equations
    ##\frac{1}{f}=\frac{1}{p}+\frac{1}{q}## - the mirror equation
    3. The attempt at a solution
    I dont know if the mirror equation can be applied here and to be honest i dont know what these information tell me...
    Focal length is given and the ##1cm## is supposed to be the ##p## in the equation, the distance of the observer to the glass, right? Im not sure of what the smalest and the largest distance of clear sight are...could you help?
  2. jcsd
  3. Jun 19, 2016 #2

    Simon Bridge

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    Look up "near point" and "far point".
    In order to see the image - what is the minimum distance it can be from his eye?
    What is the object position that will put the image there?
    It's that kind of thing.
  4. Jun 20, 2016 #3


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    The lens is to be used to form images of the small object 25 cm and very far (inifinite) from the observer.
    These are the minimal and maximal points of clear vision.
  5. Jun 20, 2016 #4

    rude man

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    There are two problems: one where the eye is accommodated to infinity and the other where it's accommodated to 25 cm.

    I suggest drawing the complete picture. This includes the magnifying lens, the eye lens and the eye retina. So you might have

    f1 = focal length of mag. lens;
    p1 = distance from object to mag. lens
    q1 = distance from mag. lens to (real) first image;
    f2 = focal length of eye lens;
    p2 = distance from 1st image to eye lens; and
    q2 = distance from eye lens to retina (where tyher second real image is finally formed.

    So, apply the mirror (actually also the lens) formula. Figure out what parameters you know and don't know (for example, you don't know f2 or q2 but what about the rest?) Your goal is to compute p1.
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