9

EXPT NO: 09.  FOCAL LENGTH OF CONVEX MIRROR

Aim :  To find the focal length of a convex mirror using convex lens.

Apparatus : Candle or illuminated object, meter scale, convex lens, convex mirror, lens stand, string, screen.

Ray diagrams :

Formula : 1)Focal length of convex mirror = f  where R is the radius of curvature of the convex mirror.

Procedure :

      Find approximate focal length of convex lens by distant object method.

      Place a convex lens in front of the convex mirror such that their principal axis coincide as shown in fig.(a).

      Illuminate an object (arrow; by source of light and place it in front of convex lens at a distance greater than focal length of lens such that its tail/head is on the principal axis. Adjust the convex mirror such that image (real, inverted and same size of the object) can be observed on the screen kept at the position of object.

      Measure the distance 'x' between the convex mirror and convex lens

      Without disturbing the object and lens remove the convex mirror, [fig. (b)]

      Place the screen on the other side of the lens and move it nearer to or away from the lens so that real, inverted and magnified image is observed on the screen.

      Measure the distance 'y' between convex lens and screen.

      The difference between these two distances (y-x) gives the radius of curvature 'R' of the mirror. Hence calculate its focal length =f= R/2.

      Repeat the experiment for different values of 'u' and find the mean focal length of convex mirror.

Observations :

1)      Approximate focal length of given convex lens =………cm

2)       Observation table :

Obs. no	Object distance 'u' cm	Distance between mirror & lens 'x' cm	Distance between screen and convex lens 'y' cm	R =+(y-x) cm	f=R/2 cm
01
02
03

Result : Focal length of given convex mirror =………………cm

Precautions :

1)      Focal length of convex mirror must be greater than focal length of convex lens.

2)      Lens should be clean.

J K L