JEE Main 2025 Jan 24 Shift 1, Physics Q3: Properties of Fluids

An air bubble inside a liquid has a single liquid surface, so the excess pressure inside it is $\frac{2T}{R}$ (not $\frac{4T}{R}$, which applies to a soap bubble with two surfaces).

Pressure balance for the air inside the bubble:

$P_{in}=P_0+\rho gh+\frac{2T}{R}$

Given $P_{in}-P_0=2100$ Pa, hence

$\frac{2T}{R}=2100-\rho gh$

Substitute $\rho gh=1000\times 10\times 0.20=2000$ Pa and $R=0.1\text{cm}=10^{-3}$ m:

$2T=(2100-2000)\times 10^{-3}=0.1{\text{N/m}}^2\cdot \text{m}$

$T=0.05\text{N/m}$.

$\therefore$ Option (4).