If the speed of light in carbon disulfide and vacuum is X and Y respectively, then

The speed of light is maximum in vacuum (c ≈ 3 × 10⁸ m/s). In any other medium, the speed of light is reduced based on the refractive index of the medium: v = c/n. Carbon disulfide has a refractive index of approximately 1.63, so the speed of light in carbon disulfide is about 1.84 × 10⁸ m/s, which is less than the speed of light in vacuum. Therefore, X (speed in CS₂) < Y (speed in vacuum).