When two soap bubbles of radii a and b b > a coalesce, the radius of curvature of common surface is:

If radius of interface is r' and radii of each bubble is r1 and r2, then equating pressure on either side of common boundary givesP0+4Tr1−P0+4Tr2=4Tr'∴1r'=1r1−1r2 ⇒1r'=r2−r1r1r2; here r1=a, r2=b ⇒1r'=b−aab⇒r'=abb−a