Aim
To find the focal length of a concave lens using a convex lens.
Apparatus
An optical bench with four upright (two fixed uprights in middle, two outer uprights with lateral movement), a convex lens (less focal length), a concave lens (more focal length), two lens holders, two optical needles (one thin, one thick), a knitting needle and a half metre  scale.
A Short Description about the Arrangement
As a concave lens always forms a virtual image, its focal length can not be found directly as for a convex lens. For this purpose, indirect method is used, as described below.
An object needle O is placed on one side of a convex lens L1 and its real inverted image I is located (by image needle) on the other side as shown in ray diagram.
The concave lens L2 is placed between convex lens L1 and image needle I. The concave lens diverges the rays and the image is now formed at I’ as shown in ray diagram.
For concave lens, I is the virtual object and I’ is the real image. Hence, O2I = u and O2I’ = v.
Focal length can be calculated, using lens formula
to-find-the-focal-length-of-a-concave-lens-using-a-convex-lens-1
Theory
to-find-the-focal-length-of-a-concave-lens-using-a-convex-lens-2
Ray diagram
to-find-the-focal-length-of-a-concave-lens-using-a-convex-lens-3
Procedure
To determine rough focal length of convex lens
  1. Mount the convex lens in lens holder.
  2. Go out in the open and face the lens towards distant tree or building.
  3. Obtain the image of the tree or the building on a white painted wall (screen) and move the lens forward and backward to get a sharp image on the wall.
  4. Measure the distance between the lens and the wall (screen). This will be equal to the rough focal length of the mirror.
    To set the convex lens
  5. Follow steps 2 to 4 of Experiment 2 To set the object needle
  6. Follow steps 5 to 8 of Experiment 2 To set the image needle at I
  7. Follow steps 21 to 27 of Experiment 2 To set the concave lens
  8. Clamp the holder with concave lens on fixed upright on the I side of the convex lens.
  9. Fix this upright at some distance away from the convex lens.
  10. Set the concave lens surface in same manner as convex lens surface with principal axes of the lenses coinciding.
    To set the image needle at I’
  11. Repeat steps 4 and 5 of the experiment.
    To get more observations
  12. Follow steps 29, 30 and 31 of Experiment 2.
Observations
to-find-the-focal-length-of-a-concave-lens-using-a-convex-lens-4
to-find-the-focal-length-of-a-concave-lens-using-a-convex-lens-5 (2)
Calculations
1. Find difference of positions of  L2 and I and write it as observed u in column 3a.
2. Find difference of positions of L2 and I’ and write it as observed v in column 36.
3. Apply index correction and write corrected values of u and v in columns 4a and 46.
4. Calculate f = uv/u-v and write in column 5.
5. Take mean of different values of as recorded in column 5.
to-find-the-focal-length-of-a-concave-lens-using-a-convex-lens-6
Result
The focal length of the given concave lens = -……cm
Precautions
  1. Focal length of the convex lens should be less than the focal length of concave lens so that the combination is convex.
  2. The lenses must be clean. .
  3. Other precautions are same as given in Experiment 3.