Solution:
Solution:
Fringe width in first case, $$\beta_{1} = \frac{D\lambda_{1}}{d}\quad...\left(i\right) $$
Fringe width in second case, $$\beta_{2}=\frac{D\lambda_{2}}{2d} \quad...\left(ii\right) $$
Divide equation $$\left(ii\right)$$ by $$\left(i\right)$$,
$$ \therefore \frac{\beta_{2}}{\beta_{1}}= \frac{D\lambda_{2}/ 2d}{D\lambda_{1} /d} = \frac{1}{2} \cdot\frac{\lambda_{2}}{\lambda_{1}} $$ or
$$\beta_{2} = \frac{1}{2}\cdot\frac{\lambda_{2}}{\lambda_{1}} \cdot\beta_{1} $$
$$\therefore \beta_{2} = \frac{1}{2}\times\frac{ 7500 Å}{6000 Å} \times 0.8 mm $$
$$= 0.5 mm $$
