The focal length (f) of a thin convex lens is determined by two-pin method. The numerical values of the image distance (v) are measured by various object distances (u). Figure 16.57 shows a graph of  $\frac{1}{v}$ (in cm^-1 ) versus $\frac{1}{u}$ (in cm^-1 ). The optical power of the lens is 

In two-pin method, the image formed is real and inverted. The focal length of a thin convex lens forming a real image is given by

$$\begin{gathered} \frac{1}{f}=\frac{1}{v}+\frac{1}{u} \\ where u, v and f are numerical values (without signs) ofu, v and f. Thus \\ \frac{1}{v}=-\frac{1}{u}+\frac{1}{f} \\ Therefore, the slope of \frac{1}{v} versus\frac{1}{u} graph is equal to - 1 and the intercept =\frac{1}{f}, i.e. \\ 0.05 =\frac{1}{f}\Rightarrow = 20 cm = 0.2m \\ \because Power of lens = \frac{1}{2}= +5 D, \end{gathered}$$