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Homework Help: Short optics question

  1. Mar 30, 2008 #1
    [SOLVED] Short optics question

    Shiny lawn spheres placed on pedestals are convex mirrors. One such sphere has a diameter of 42 cm. A 13 cm robin sits in a tree 2.1 m from the sphere.
    (a) Where is the image of the robin? in cm
    b) How long is the robin's image? in cm


    Ok, I know that radius of a concave lens is 2f but when i use that principle i do not get the right answer...what i did is:
    21/2=10.5=focal length
    x=11.0526 which was my answer for a which webassign says is wrong
    once i get this answer part b is very easy as all i will have to do is x/13=-answer to part a/210
    plz help me figure out what i did wrong im assuming that radius of a convex lens which is what is used in problem is not 2f but i have no clue what it is then and i tried googling it but could not find anything...thanks
  2. jcsd
  3. Mar 30, 2008 #2


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    Homework Helper

    For a convex mirror, f (and R) are negative.
  4. Mar 30, 2008 #3
    wow i got it thanks so much
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