A diffraction grating is in an empty aquarium tank. The index of refraction of the glass walls is $.$ A helium$-$neon laser is outside the aquarium. The laser beam passes through the glass wall and illuminates the diffraction grating.

What is the first$-$order diffraction angle of the laser beam?

What is the first$-$order diffraction angle of the laser beam after the aquarium is filled with water $?$