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EXPT NO: 07. FOCAL LENGTH OF CONCAVE MIRROR

Aim :  To find the value of v for different values of u in case of a concave mirror and to find the focal length.

Apparatus : Concave mirror, screen, index pins, meter scale.

Ray diagrams :

Formula : 

f    =  focal length of concave lens

u    =  object distance for a concave lens,

v    =  image distance for a concave lens.

Procedure :

            Using concave mirror, focus the image of distant object on the screen. Note the distance between screen and mirror which is approximate focal length ''.

            Keep the index pin in front of concave mirror at a distance of about Adjust the pin in such a way that the image of pin coincides with itself without parallex .Note the object distance  and the image distance .

            Choose object distance between  and  adjust the image pin such that there is no parallex between image of object pin seen in mirror and image pin. Measure  and  .

            Take two reading for   between   and  .Take two reading for   greater then 

            Place the object pin within distance  .In this case virtual image is obtained .Adjust the image pin behind the mirror  and remove parallax  between image of an object pin and upper part of image of pin. Note the value of u and v .Repeat for one more value less than .

6)       calculate the focal length of the concave mirror.

Observations and Calculations :

Approximate focal length of concave lens =   f =  …………………. cm.

Obs.No	Position of object	cm	cm	cm-1	cm-1		cm	Mean   cm
01
02
03
04
05
06
07

Calculations:

Result : Focal length of convex lens f=…………………cm

Precautions :

      Measure the object distance and image distance accurately.

      Align properly the object pin, the image pin and the convex lens.

      Measure object distance and image distance parallel to the principal axis of the lens.

J K L