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

The speed of light in a vacuum (Y) is the maximum possible speed at which light can travel, approximately 3!5 10^8 m/s. The refractive index (n) of a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium (n = Y/X). For any material medium, the refractive index is always greater than one because light travels slower in matter than in a vacuum [3]. Specifically, carbon disulfide has a refractive index of approximately 1.63 [2]. This implies that the speed of light in carbon disulfide (X) is 1.63 times slower than in a vacuum (Y). Therefore, the speed of light in carbon disulfide (X) is strictly less than the speed of light in a vacuum (Y), making X < Y the correct relationship.

Sources

1. [3] https://www.sciencedirect.com/topics/engineering/index-of-refraction
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 149