Physics - Wave Optics Question with Solution | TestHub

When chamber is evacuated, the refractive index ( $\mu$ ) decreases. Therefore, wavelength $\left(\lambda \propto \frac{1}{\mu }\right)$ increases. Now, fringe width is given by $\begin{array}{rl}& \beta =\frac{\lambda D}{d}\\ \Rightarrow & \beta \propto \lambda \end{array}$ Therefore, as wavelength increases, the fringe width will increase.