S1 and S2 are two hollow concentric sphere enclosing charges Q1 and 2Q How will the electric flux through the sphere S1 change if a medium of dielectric?

When a dielectric medium is introduced between the two concentric spheres S1 and S2, the electric flux through S1 will remain unchanged. This is because the electric flux is determined by the charge enclosed within the surface, as described by Gauss's law. Since S1 encloses only charge Q1, the electric flux through S1 is solely dependent on that charge, irrespective of the presence of the dielectric medium. The dielectric affects the electric field within the medium but does not alter the total enclosed charge.