You have a lens .suggest an experiment to find out the focal length of the lens
Answers
Aim
To find, the focal length of a convex lens by plotting graphs between u and v or between 1/u and 1/v.
Apparatus
An optical bench with three uprights (central upright fixed, two outer uprights with lateral movement), a convex lens with lens holder, two optical needles, (one thin, one thick) a knitting needle and a half metre scaleTheory
The relation between u, v and f for a convex lens is
to-find-the-focal-length-of-a-convex-lens-by-plotting-graphs-between-u-and-v-or-between-1u-and-1v-1
where,
f = focal length of convex lens
u = distance of object needle from optical centre of the lens
v = distance of image needle from optical centre of the lens.
Note. According to sign-convention, u has negative value and v has positive value. Hence, f comes positive.
Ray diagram
to-find-the-focal-length-of-a-convex-lens-by-plotting-graphs-between-u-and-v-or-between-1u-and-1v-2
Procedure
To determine rough focal length
Mount the concave mirror in mirror holder.
Go out in the open and face the mirror towards distant tree or building.
Obtain the image of the tree or the building on a white painted wall (screen) and move the mirror forward and backward to get a sharp image on the wall.
Measure the distance between the mirror and the wall (screen). This will be equal to the rough focal length of the mirror.
To set the lens
Clamp the holder with lens in a fixed upright and keep the upright at 50 cm mark.
Adjust the lens such that its surface is vertical and perpendicular to the length of the optical bench.
Keep the upright fixed in this position throughout.
To set the object needle
Take the thin optical needle as object needle (O). Mount it in outer laterally move¬able upright near zero end.
Move the object needle upright and clamp it at a distance (in full cms) nearly 1.5 times the obtained rough focal length of the lens.
Adjust height of the object needle to make its tip lie on horizontal line through the optical centre of the lens.
Note the position of the index mark on the base of the object needle upright.
To set the image needle
With left eye closed, see with the right open eye from the other end of the optical bench. An inverted and enlarged image of the object needle will be seen. Tip of the image must lie in the middle of the lens.
Mount the thick optical needle (image needle) in the fourth upright near the other end of the optical bench.
Adjust the height of the image needle so that its tip is seen in line with the tip of the image when seen with right open eye.
Move the eye towards right. The tips 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 the image needle upright.
Record the position of the index marks on the base of upright of the lens, the object needle and the image needle in the table against observation 2.
To determine index correction
Find the index correction for distance between optical centre of lens and tip of the object needle and also for distance between optical centre of lens and tip of the image needle as described.
To get more observations
Move object needle upright towards mirror in steps of 1 cm to get observation 2 and 1. Repeat the experiment.
Move object needle upright away from mirror (from position of observation 2) in steps of 1 cm to get observations 4, 5 and 6. Repeat the experiment.
Record all the observations as given ahead.
(Note. Same as in Experiment 1).
Observations
Rough focal length of the given convex lens = …….cm
Actual length of the knitting needle x=…….cm
Observed distance between the object needle and the lens
when knitting needle is placed between them y =…….cm
Observed distance between the image needle and the
lens when knitting needle is placed between them z =…….cm
Index correction for the object distance u, x -y =…….cm
Index correction for the image distance v, x-z =…….cm
Answer:
Aim: To find the focal length of a lens
Apparatus required: double convex lens, metre scale, a white screen, lens holder ,distant object
Procedure:
• Set up the apparatus near a window so to locate a distant object .
• Mount the convex lens on the lens holder.
• Now place the metre scale on the table and keep the lens at a distance of 5 cm mark on the metre scale such that the mid point of lens holder coincides with 5 cm mark.
• Place the white screen behind the lens such that the image of the object is formed on the screen.
• Now move the screen until you get a sharp, diminished & inverted image of the distant object.
• Mark the point on the scale where you get the desired image. The difference between the lens mark & the screen mark will give the focal length.
• Now place the lens on a new position that is 10 cm and find the focal length.
• Repeat the above steps by keeping the lens at 10 cm, 15 cm & so on to get more observations
Observation: