Final answer:
In the Young's double-slit experiment, the fringe separation on a screen would not change if all the air was pumped out and replaced by a vacuum since the wavelength of the light and slit separation remain constant.
Step-by-step explanation:
When discussing the Young's double-slit experiment, the interplay between the wavelength of light, the slit separation, and the medium through which the light travels (air, vacuum, water, etc.) is crucial to understanding the resulting interference pattern on a screen.
In the given scenario, if all the air was pumped out, creating a vacuum, the wavelength of light would remain constant, as it is characteristic of the light source. However, because air and vacuum have different refractive indices, with vacuum having a refractive index of approximately 1, the light would experience less optical path difference than in air. This would result in no change in fringe separation since the separation is determined by the ratio of the wavelength of light to the slit separation, and neither of these factors would have been altered by the change of medium.