Solution:
Fringe width, \(\beta=\frac{\lambda D}{d}\)
\(\beta^{\prime}=\frac{\lambda D}{d^{\prime}}\)
Now, \(d^{\prime}=\frac{d}{2}\) and
\(D^{\prime}=2 D\)
So, \(\beta^{\prime}=\frac{\lambda \times 2 D}{d / 2}\)
\(=\frac{4 \lambda D}{d}\)
\(\beta^{\prime}=4 \beta\)
Fringe width becomes 4 times