Which one of the following statements about the refractive index of a material medium with respect to air is correct ? -

The refractive index (n) of a material medium with respect to air is defined as the ratio of the speed of light in air (c) to the speed of light in that medium (v) [2]. Since light travels at its maximum speed in a vacuum and nearly the same speed in air, the speed of light in any other material medium is always less than its speed in air [3]. Consequently, the ratio n = c/v results in a value that is always greater than one for all common transparent materials [2]. For instance, the refractive index of water is approximately 1.33 and glass is 1.50 [4]. While advanced research into metamaterials has explored negative or zero refractive indices, these are artificial structures and not standard material media encountered in classical optics [5]. In the context of standard physical science, the refractive index of a medium relative to air is consistently greater than unity.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 148
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 149
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159
4. [4] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > QUESTIONS > p. 150
5. [5] https://www.sciencedirect.com/science/article/abs/pii/S0038109808002184