Which one of the following statements is not correct for light rays?

Light travels at different speeds depending on the optical density of the medium [5]. In a vacuum or air, light travels at its maximum speed of approximately 300 million meters per second (3 × 10⁸ m/s) [2]. When light enters a denser medium like water (refractive index ≈ 1.33) or glass (refractive index ≈ 1.50), its speed reduces significantly [4]. Conversely, when light leaves a denser medium and enters a rarer medium, it speeds up. Therefore, light speeds up as it leaves a glass surface to enter the air [5]. Statement 3 is incorrect because water is optically denser than air; thus, light speeds up (rather than slows down) as it leaves the water surface and enters the air [4]. This change in speed is the fundamental cause of refraction [5].

Sources

1. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159
2. [5] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 147
3. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 148
4. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > QUESTIONS > p. 150
5. [4] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 149