Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Lens query.

  1. Feb 7, 2010 #1
    Hi, in questions involving lenses, when using the "thin lens formulae"
    if my di is negative doesnt that mean the image is on the other side of the lens?
    in this example however it doesn't seem this holds true..

    A doctor examines a patient's skin lesion with a 15 cm focal length magnifying glass. If this lens is held 13.5 cm from the lesion, where is the image and could it be projected onto a screen?

    the answer is;
    1.35 m on the patient's side of the lens and could not be projected onto a screen.

    I get how to get 1.35 but i get -1.35 so shouldn't it mean that it would be on the doctors side?

    can somone please explain this to me.

  2. jcsd
  3. Feb 8, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi luysion! :smile:

    Distance is positive for real objects and images, and negative for virtual ones.

    The light is coming from the patient …

    if it was focussing onto a real image, it would be on the doctor's side of the lens (where you could put a screen) …

    since it's on the patient's side, it's virtual and negative, which is why you get -1.35. :wink:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook