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 (+)
- 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 .
- Mount the thick optical needle in the fourth upright near the other end of the optical bench .
- Adjust the height of the image needle (thick) so that its tip is seen in line with the tip of the image when seen with the right eye open .
- Move the eye towards right .The tip will get separated .The image tip and the image needle tip have parallax .
- Remove the parallax tip to tip .
- Note the position of the index mark on base of image needle .
- Record the position of the index mark on the base of upright of lens , the object and image needle .
- Find index correction (if any ) as described for distance between optical center of lens and tip of image or object needle .
- Move object needle upright towards mirror in steps of 1 cm to get observation .Repeat the experiment .
- Move object needle upright towards mirror in steps of 1 cm to get the rest of the observation . Repeat the experiment .
- Record your observation .
F =uv /u-v f from u-v graph
Focal length of convex lens
|S No||Position of lens (cm)||Object length u (inch)||Image lens v
|Position of object||Position of image||1/u||1/v|
Total = 0.312 cm 0.0167cm
- Tip of the object and image needle should lie at same height as the centre of the lens .
- Parallax should be removed from tip to tip.
Sources of error
- The upright may not be vertical .
- Parallax removed may not be perfect .
Define and give lens formula .
The equation relating the object distance (u) , the image distance (v) and lens focal length (f) is called lens formula 1/u+ 1/v = 1/f.
What are the various assumption made in deriving lens formula ?
The following assumption are made :
The lens is thin .
The lens has a small .
The point object lies on to the principal axis and placed perpendicular
The incident ray make small angles with the lens surface on the principal axis .
Which convex lens has more focal length , thick or thin ?
A convex lens has more focal length .
Define optical centre of a lens ?
It is a fixed point inside the lens on its principal axis through which light ray passing undeviated .
What is the principal axis of a lens ?
The straight line passing through the centre of curvature of the curved surfaces of the lens is the principal axis of a lens .