Answer:
1.2 mm
Explanation:
Distance of 3 rd order fringe from centre
=3\frac{\lambda D}{d}
where λ is wave length of light D is distance of screen and d is separation between two lines
So distance between 3 rd order fringe of two wavelengths
3\frac{D}{d}\left ( \lambda _1-\lambda _2 \right )\\
=\frac{3\times 1\times 2000}{1\times 10^{-2}}\left ( 579-577 \right )\times 10^{-9}m\\
=12\times 10^{-1}mm
