In Young's double slit experiment, sodium light composed of two wavelengths ${\lambda }_1$ and ${\lambda }_2$ close to each other (with ${\lambda }_2$ greater than ${\lambda }_1$ ) is used. The order up to which the fringes can be seen on the screen is given by:

If an interference experiment is performed using two wavelengths close to each other, two interference patterns corresponding to the two wavelengths are obtained on the screen. The fringe system remains distinct up to a point on the screen where the n^th order maximum of one wavelength falls on the n^th order minimum of the other wavelength. Thus, interference pattern can be seen up to a distance $y_n$ from the centre of the screen if
$y_n=\frac{n{\lambda }_{1}D}{d}=\left(n-\frac{1}{2}\right)\frac{{\lambda }_{2}D}{d}$
or, $n{\lambda }_1=\left(n-\frac{1}{2}\right){\lambda }_2$ 

or, $2n{\lambda }_1=(2n-1){\lambda }_2$
which gives
$n=\frac{{\lambda }_2}{2\left({\lambda }_2-{\lambda }_1\right)}$