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Focal Length of Convex Lens

  1. Jan 1, 2012 #1
    I am trying to find the focal length of a convex lens, so i let the sun shine through my window to create an image on my wall. Is that image the focal point or is it just the distance of the image? And if it is the distance of the image, how do I put it into the thin lens equation, since the distance of the object is infinite?
  2. jcsd
  3. Jan 1, 2012 #2
    Since the sun is so far away, the distance you measure from the lens to the sun's image is equal to the focal length.
  4. Jan 1, 2012 #3
    Im pretty sure the sun's image needs to be focused doe. I think. then you measure the distance between the projection and the lens.
  5. Jan 1, 2012 #4
    Convex lenses have a focal point in front of the lens. If the distance from what you are focusing is shorter than the focal point, then you end up with a virtual image on the same side as you're object. If the distance is greater than the focal point, you end up with a real image that's projected upside down. Focusing the sun, you wouldn't notice the upside down aspect, so I would focus something not symmetrical, so you can tell. The point when the image changes from real to virtual is the focal point. Just move the lens back and forth until it changes.
  6. Jan 1, 2012 #5
    thanks :D
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