10

EXPT NO: 10. FOCAL LENGTH OF CONCAVE LENS USING CONVEX LENS

Aim :    To determine the focal length of a concave lens using convex lens.

Apparatus : Concave lens, convex lens, lens stands, screen, illuminated object, meter scale etc.

Ray Diagram :

Formula :    for concave lens            

where u = a - d.

a   = image distance for a convex lens.

d   =  distance between the convex lens and concave lens.

f    =  focal length of concave lens

u    =  object distance for a concave lens,

v    =  image distance for a concave lens.

Procedure : I

      Determine the approximate focal length of a given convex lens by using distant object.

      Keep the distance between the object and the screen greater than four times the focal length of the convex lens.

      Introduce the convex lens between object and screen, adjust the height of object and height of the convex lens properly.

      Adjust the position of the convex lens i.e. object distance so as to obtain a real, inverted and diminished image on the Screen.

      Measure the distance between, a convex lens and the screen i.e. image (say 'a')

      (Note : Keep 'a' constant throughout the experiment i.e. do not change the position of convex lens from object).

      Introduce the given concave lens between convex lens and the screen. The diminished image acts as the virtual object for the concave lens.

      Measure the distance between a convex lens and concave lens (say 'd'). Find object distance u for concave lens i.e. u = a - d.

      Shift the screen away from concave lens so as to obtain a clear and sharp image which is magnified with respect. to virtual object.

      Measure the image distance (v) between concave lens and the screen.

      For three more values of 'd' find u and v for concave lens.

      (Note : the distance 'd' should not be equal or greater than 'a')

      Using sign convention find focal length of concave lens for four different readings. Then find mean focal length of concave lens.

Observations :

i)      Approximate focal length of convex lens =……………..cm.

ii)     Distance between convex lens and diminished image, a =……………..cm.

Ob.No.	cm	cm	cm	cm	Mean  cm
01
02
03
04

Calculation:              

(use log table to find reciprocal of four fig numbers).

Ob.No.	cm-1	cm-1	cm-1	cm	Mean  cm
01
02
03
04

Result : Focal length of concave lens =…………cm

Precautions :

      Select the convex lens and concave lens so that the combination acts as converging lens.

      The height of object, convex lens and concave lens should be adjusted properly. The plane of object, convex lens, concave lens and screen should be parallel.

      Measure u and v parallel to principal axis of the lens.

      Observe well defined, sharp image on the screen.

J K L