When a ray of light is going from one medium to another, its :

When a ray of light travels from one transparent medium to another, it undergoes refraction due to a change in its speed of propagation [c1][c2]. According to the wave equation v = fλ, where v is speed, f is frequency, and λ is wavelength, a change in speed must result in a change in either frequency or wavelength [t1][t5]. Scientific evidence confirms that frequency is an intrinsic property determined solely by the light source and the electromagnetic oscillation rate, which must remain continuous across the interface [t1][t4]. Consequently, while the speed and wavelength of light alter proportionally to the refractive index of the medium (λ = v/f), the frequency remains constant [t2][t8]. For instance, when light enters a denser medium like glass from air, its speed and wavelength decrease, but its frequency stays the same [t1][t9].

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 > Activity 9.10 > p. 147
3. [3] https://www.ncbi.nlm.nih.gov/sites/books/NBK592413/