AIM : To determine the focal length of a convex lens by obtaining the image of a distant object.
APPARATUS : A Convex lens , optical bench , two needles , knitting needle , three uprights and a metre scale.
THEORY : When an object which placed between f and two f where f is the rough focal length of convex lens then its real inverted and magnified image is formed beyond to F on the other side of the lens as shown in figure the focal length of the convex lens can be calculated by the using the lens formula.
1/f = 1/v - 1/u , where u is the object distance and v is the distance of image from the lens.
Observations:
Rough focal length of the convex lens , f = (i) ....... cm
Actual length of the knitting needle , x =
Observed distance between the convex lens and object needle AB , y = ....cm
Observed distance between the convex lens and image needle A'B' , z = ....cm
Index correction for u , (x-y) = ....cm
Index correction for v , (x-z) = ....cm
CALCULATIONS :
1. u = ........... cm, v = ....... cm
Using the formula , 1/f = 1/v - 1/u , we get
f1 = uv/u-v = ..... cm
2. u = ........... cm, v = ....... cm
Using the formula , 1/f = 1/v - 1/u , we get
f2 = uv/u-v = ..... cm
3. u = ........... cm, v = ....... cm
Using the formula , 1/f = 1/v - 1/u , we get
f3 = uv/u-v = ..... cm
Result : Focal length of given convex lens = ...... cm
Precautions :
1. The uprights should be placed vertical
2. The tips of the needles should be in line with the optical centre of convex lens.
3. The eye should be kept at a distance of about 25 cm from the upright in which image needles is inserted.
4. Index correction should be determined and applied carefully.
5. The parallax should be removed tip to tip.
6. While recording the rough focal length a sharp image of the distant object should be obtained.
Source of Error :
1. The uprights may not be vertical
2. The principal axis of the lens may not be parallel to the optical bench scale.