Can interference take place if the medium between the slits and screen is changed? In Young's double-slit experiment, the medium between the slits and screen is usually air. But, if we replaced air with another medium, for example water, the wavelength of light would change and the fringe separation should also change as it is equal to wavelength times distance from slits to screen, divided by distance between slits. But will it be possible to observe an interference pattern in such a case?
 A: Yes, as long as the medium does not prevent the light from passing or scatter the light too much. If there is scattering of the light as it passes through the medium, the phase of the light will be scrambled, this reduces the fringe visibility and in severe cases will destroy the interference pattern.
In fact, standard interferometers such as the Mach-Zehnder interferometer separate the beam paths into two, if one of the two beams is passed through an object with a different refractive index than the medium, a phase shift of the beam will occur relative to the second beam. This results in a phase shift of the interference and provides information about the properties of the object in the path.
A: Yes it will be possible to observe an interference pattern, provided the light can travel through the medium (like water) without too much scattering. The water will not destroy the interference pattern, since there will still be a path difference between the rays. There is no reason why a medium like water, will completely destroy the interference pattern.
Rather, since the light will refract in the new medium and will have a  new wavelength in the medium given by $$\lambda ' = \frac{\lambda}{n}$$  the fringes will be shifted and closer together  ($n\gt 1$ of course).
