Calculate the focal length of convex lens observation table
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Aim : To find the focal length of a convex lens by plotting graph between u and v.
Apparatus : An optical bench with three upright (central upright fixed , two outer uprights with lateral movements ), a convex lens with lens holder , two optical needle , (one thin , one thick ) a knitting needle and a half meter scale .
Theory : The relation between u , v and for a convex lens is
1/f= 1/v – 1/u Where f=focal length of convex lens
i/f = u-v/uv u =distance of object needle from optical center of the lens (-)
f =uv/u-v v = distance of image needle from optical from optical center of lens (+)
Procedure
Mount the concave mirror in the mirror holder .
Obtain the image of a tree or building outside of the window on a screen and move the mirror forward and backwards to get a sharp image on the screen .
Measure the distance between mirror and screen (rough focal length ).
Clamp the holder with lens fixed upright at 50cm mark .
Adjust the lens so that its surface is vertical and upright keep it fixed .
Take the thin optical needle as object needle (o).Mount it in outer laterally moveable upright near zero end .
Move the object needle upright and clamp it at a distance nearly 1.5 times the rough focal length obtained earlier .
Adjust height of the object needle to move its tip lie on a horizontal line through the optical center of the lens .
Note the position of the index mark on the base of object needle .
With the eye closed , see with the right eye from the other end of the optical bench . An inverted and enlarged image of the object needle will be seen .
Result
F =uv /u-v f from u-v graph
Diagram
Focal length of convex lens
Observation table
Apparatus : An optical bench with three upright (central upright fixed , two outer uprights with lateral movements ), a convex lens with lens holder , two optical needle , (one thin , one thick ) a knitting needle and a half meter scale .
Theory : The relation between u , v and for a convex lens is
1/f= 1/v – 1/u Where f=focal length of convex lens
i/f = u-v/uv u =distance of object needle from optical center of the lens (-)
f =uv/u-v v = distance of image needle from optical from optical center of lens (+)
Procedure
Mount the concave mirror in the mirror holder .
Obtain the image of a tree or building outside of the window on a screen and move the mirror forward and backwards to get a sharp image on the screen .
Measure the distance between mirror and screen (rough focal length ).
Clamp the holder with lens fixed upright at 50cm mark .
Adjust the lens so that its surface is vertical and upright keep it fixed .
Take the thin optical needle as object needle (o).Mount it in outer laterally moveable upright near zero end .
Move the object needle upright and clamp it at a distance nearly 1.5 times the rough focal length obtained earlier .
Adjust height of the object needle to move its tip lie on a horizontal line through the optical center of the lens .
Note the position of the index mark on the base of object needle .
With the eye closed , see with the right eye from the other end of the optical bench . An inverted and enlarged image of the object needle will be seen .
Result
F =uv /u-v f from u-v graph
Diagram
Focal length of convex lens
Observation table
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