A-ray of light travelling from a rarer medium to a denser medium

When a ray of light travels from an optically rarer medium to an optically denser medium, it undergoes a change in speed and direction due to the difference in refractive indices. Light travels fastest in a vacuum and slows down when it enters a medium with a higher optical density [2]. According to Snell's Law, this reduction in speed causes the light ray to bend towards the normal line at the interface of the two media [5]. Conversely, when light moves from a denser to a rarer medium, it speeds up and bends away from the normal [3]. Therefore, the correct behavior for a ray entering a denser medium from a rarer one is that it slows down and bends towards the normal [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 149
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 148
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 147
4. [5] https://www.ncbi.nlm.nih.gov/books/NBK594230/