Explanation

(c) Excess pressure inside a soap bubble of radius R is
$= \frac{4T}{R}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ldots\ldots(i)$
Where T is surface tension of liquid film.
Pressure due to oil column
$= h\rho g\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ldots\ldots.(ii)$
Where $h$ is height of column, $\rho$ the density and $g$ the gravity.
From Eqs. (i) and (ii), we get
$\frac{4T}{R} = h\rho g$
$\Longrightarrow \ \ T = \frac{h\rho gR}{4}$
$Given,\ \ \ \ \ h = 2\ mm = 0.2\ cm,\ \ \ g = 980{cms}^{- 2},$
$\rho = 0.8\ gcc^{- 1},\ R = 1\ cm$
$\therefore\ \ T = \frac{0.2 \times 0.8 \times 980}{4}$
$= 3.92 \times 10\ dyne\ {cm}^{- 1}$
$In\ {Nm}^{- 1}$
$T = 3.9 \times 10 \times \frac{10^{- 5}}{10^{- 2}} = 3.9 \times 10^{- 2}\ {Nm}^{- 1}$