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

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    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

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    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.
     
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