A composite parallel plate capacitor is made up of two different dielectric materials with different thickness $$\left(t_{1}\right.$$ and $$\left.t_{2}\right)$$ as shown in figure. The two different dielectric materials are separated by a conducting foil $$\mathrm{F}$$. The voltage of the conducting foil is V.

Resistances are connected in a meter bridge circuit as shown in the figure. The balancing length $$l_{1}$$ is $$40 \mathrm{~cm}$$. Now an unknown resistance $$x$$ is connected in series with $$\mathrm{P}$$ and new balancing length is found to be $$80 \mathrm{~cm}$$ measured from the same end. Then the value of $$x$$ will be ____________ $$\Omega$$.

The effective current I in the given circuit at very high frequencies will be ___________ A.

The graph between $$\frac{1}{u}$$ and $$\frac{1}{v}$$ for a thin convex lens in order to determine its focal length is plotted as shown in the figure. The refractive index of lens is $$1.5$$ and its both the surfaces have same radius of curvature $$R$$. The value of $$R$$ will be ____________ $$\mathrm{cm} .$$ (where $$u=$$ object distance, $$v=$$ image distance)