DATE:PRACTICAL #:TITLE: Focal Length of Convex LensAIM: To find the focal length of a convex lens by the plotting of graphs between u an v and between 1/u and 1/v.Apparatus/Materials: Convex lens, screen , illuminated wire gauze, ruler. DIAGRAM:THEORY: Aconverging lensis a lens that converges rays of light that are traveling parallel to its principal axis. The principal axis refers to the line of symmetry across the lens. The focus is the point through which light rays striking the lens along a line close to and parallel to the principal axis pass.

The focal length is a fixed property of a lens and if an object is palced at a distance u from a lens of focal length, f, then an image of the object will be focused in the lens at a distance v from it according to the equation

Procedure:Method 1(Distant/Object method). The convex lens is faced to a distant scenery and the positon of the screen is adjusted so that a well defined image of the scenery(object) is formed on the screen. The distance between the lens and the image on the screen give the approximate focal length. The experiment is repeated and the mean focal length is calculated.Method 2The convex lens is placed in front of the illuminated wire gause. A screen is adjusted on the other side of the lens so that a clear image of the object is formed on it. The distance of the object and the image from the lens are measured as u and v . the focal length is then calculates using the formula

The experiment is repeated for different values of u. readings are taken for both magnified and diminished images.

Draw the table given in the class. That is the formation of image by convex lens for different object positionsEg. Image at infinity Beyond 2F etc. (you must add table here showing 6 positions of image)Method 3u-v graph ( no minus sign between u and v, hyphen is shown between u and v) ( you must use the data from method 2 to find out f)a graph is drawn with u along the X- axis and v along Y-axis TAKING A COMMON ORIGIN AND SAME SCALE FOR BOTH AXIS . A bisector to the angle YOX is drawn which meets the graph at P. then OA=OB= 2f. thence focal length can be found out.Method 41/u - 1/v graph ( between 1/u and 1/v , hyphen(-) is shown) A graph is drawn with 1/u along the X axis an 1/v along Y axis taking zero as origin and same scale for both axes. The graph is a straight line intercepting the axes at A and B . then OA=OB= 1/f. hence f can be calculated.

Data collectionTo find focal lengthMethod 1 ( distant object method)Trial 1 focal length f =----------------Trial 2 focal length f=-------------------Trial 3 focal length f=--------------------Mea

n focal length=--------------------------

METHOD 3 (U V GRAPH)Trial Object distanceu /cmImage distance v/cm

1.14.3

35.4

2.15.528.7

3.16.824.3

4.21.219.5

5.22.318.5

23.017.3

Method 3 ( u-v graph)Trial u/v/

12345

Method 4 ( 1/u-1/v graph)Draw table above( method 3) and add two more columns for 1/u and 1/v . give answers for correct signifincant figuresResultFocal length of convex lens1. By distant object method=-------------2. By u-v method =---------------3. From u-v graph =---------------4. From 1/u-1/v graph =---------------