\(\beta=0.3\; cm . D=1\; m =100 \;cm\)

Distance between images of the sources \(0.8 \;cm\)

Distance of images from lens, \(v=80 \;cm\)

Distance of slit from lens \(= u\)

\(\frac{1}{v}+\frac{1}{u}=\frac{1}{f}\)

\(\Rightarrow \frac{1}{60}+\frac{1}{u}=\frac{1}{16}\)

\(\Rightarrow u=20\; cm\)

Magnification \(=\frac{v}{u}=\frac{80}{20}=4\)

Magnification \(=\frac{\text { Distance between images of slits }}{\text { Distance between slits }}\) \(=\frac{0.8}{d}=\frac{0.8}{d}=4\)

\(\Rightarrow d=0.2\; m\)

\(\Rightarrow \beta=\frac{D \lambda}{d}=\frac{100 \lambda}{2}=0.03\)

\(\Rightarrow \lambda=6000 \;\mathring A\)