Determining focal length of a converging lens
Answer Text: Determining focal lengths:1. Determining focal length of a converging lensExperiment: To determine the focal length of a converging lens using the lens formula.Procedure.1. Set up the apparatus as shown below2. Place the object at reasonable length from the screen until a real image is formed on the screen. Move the lens along the metre rule until a sharply focused image is obtained.3. By changing the position of the object obtain several pairs of value of u and v and record your results as shown. Discussion:The value #frac{u v}{ u + v} #is the focal length of the lens and the different sets of values give the average value of âfâ.Alternatively the value âfâ may be obtained by plotting a graph of #1 / v# against #1 / u#.When plotted the following graph is obtained.Since# 1 / f = 1 / u + 1 / v#, at the y-intercept# 1 / u #= 0, so that #1/ f = 1 / v# or f = v.The focal length may therefore be obtained by reading off the y-intercept and finding the reciprocal. Similarly at the x-intercept,# 1 / v# = 0, therefore #1 / f = 1 / u# or f = u hence the focal length can also be obtained by reading off the x-intercept and finding the reciprocal.