\(\frac{ y }{ L }=\theta=\frac{3 \lambda}{2 a } \quad L =2 m\)

\(y _{1}=\frac{3 \lambda_{1} L }{2 a } \quad \lambda_{2}=655\, nm\)

\(y _{2}=\frac{3 \lambda_{2} L }{2 a } \quad \lambda_{1}=650\, nm\)

\(a=0.5 \,nm\)

\(\Delta y = y _{2}- y _{1}=\frac{3\left(\lambda_{2}-\lambda_{1}\right)}{2 a } L\)

\(=\frac{3(655-650)}{2 \times 0.5 \times 10^{-3}} \times 2 \times 10^{-9}\)

\(=\frac{3 \times 5 \times 2}{1 \times 10^{-3}} \times 10^{-9}\)

\(=3 \times 10^{-5}\)