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Concave mirror and virtual image

  1. Oct 30, 2006 #1
    we used a concave mirror and a candle for an experiment
    when the candle wa placed between the vertex and the focus of the mirror (f=12.7cm) a virtual image appeared behind the mirror.
    We measure the image distance approximately by pointing a finger towards where the image might be and got 6.0cm (-6.0 when used in mirror equation) and the object was at 6.35cm. However, when I use the mirror equation to calculate the focus, i get a negative answer that is not even close to the actual focus 12.7cm. Do u think this is just due to human errors of not measuring the image distance accurately or is there some other factors that can account for this? OR is the image distance supposed to be almost the same as the object distance in this case?
  2. jcsd
  3. Oct 30, 2006 #2


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    Staff: Mentor

    For the given focal length (12.7) and object distance (6.35), you should get an image distance of -12.7.

    So you must have either put the object in the wrong location, or else measured the image distance incorrectly. It sounds like you used the "parallax method" to locate the image. That is, you moved your finger around until you found a location where it appeared to line up with the image no matter what angle you looked at them from. I find that many students have trouble with this at first.
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