If vacuum is created in the room where Young's double slit experiment is performed. Then:

If vacuum is created in the room where Young's double slit experiment is performed, then the wavelength of the light will be slightly higher than that in air as per the relation below:

$\mathrm{\lambda } = \frac{\mathrm{v}}{\nu }.$

The speed of light c is slightly greater than that in air, therefore, the wavelength of light will also be greater in vacuum than that in air.

Since fringe width is directly proportional to the wavelength of the light, fringe width will be higher in this case.