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

Focal length of concave lens using convex lens

  1. Jun 9, 2006 #1
    # A convex lens of focal length 10 cm forms a real image of an object placed at a distance of 20 cm from it. Midway between the convex lens and the position of the image a thin concave lens is introduced. The image formed now is at a distance of 5 cm away from the earlier position. What is the focal length of the concave lens?
    I solved it in the following way:
    Case-1
    Let u, v and f be the object distance, image distance and the focal length of the convex lens.
    Here u = 20 cm, f = + 10 cm
    1/v = 1/f – 1/u
    1/v= 1/10 – 1/20
    v = 20 cm
    Case- 2
    Let u, v and f be the object distance, image distance and the focal length of the concave lens.
    Here u = - 10(virtual object), v = + 15 cm
    1/f = -1/10 + 1/15
    f = - 30 cm
    But the answer given in my book is – 5 cm.
     
  2. jcsd
  3. Jun 11, 2006 #2

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I don't get 5 cm either.

    But I think you made a mistake with setting v=15cm in the second part. Would you expect the image for a concave lens to be 5cm closer to the lens or 5cm farther from it (compared to the object)?
     
  4. Jun 12, 2006 #3
    I assumed it to be away because i felt that concave lens is diverging in nature and so the image would be produced 5 cm away from the lens. What do you say?
     
  5. Jun 13, 2006 #4

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That sounds right (that the image would be farther away). There must be a mistake with the way I was doing it. I'm too sleepy to think straight now. I'll look at this again when I find some free time. Meanwhile someone else might have something useful to say.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Similar Discussions: Focal length of concave lens using convex lens
Loading...