Dipping the frame in a soap solution forms a rectangular film of length $0.02\,\text{m}$ and height $h = 0.030\,\text{m}$. White light falls on the film at an angle of $30^\circ$ (measured with respect to the normal). The reflected light displays green colour of wavelength $\lambda_0 = 500\,\text{nm}$. If the density of soap solution is $1000\,\text{kg/m}^3$ and $\mu_{\text{soap}} = 1.33$, find the mass of the thinnest film.