Question

To find the focal length of a convex lens by plotting graphs between u and v or between 1/u and 1/v

Answer 1

You should plot the graph between 1/u and 1/v.

The focal length of the convex lens is given by the relation

1/f=1/u+1/v

Now if you plot the graph between u and v you will not get the desired focal length of the lens, as u and v are not directly proportional to each other. therefore, you should plot the graph between 1/u and 1/v, to obtain the focal lengh of the lens.