Physics

Light's frequency is determined by the source that emits it and doesn't change when light enters a new medium—the oscillations per second stay the same. However, light slows down in glass (due to interaction with atoms), and since $v = f\lambda$, a lower speed with constant frequency must mean a shorter wavelength. Think of it like a marching band entering mud: the beat (frequency) stays the same, but the soldiers move slower and bunch closer together (shorter spacing, or wavelength).

Light's frequency is determined by the source that emits it and doesn't change when light enters a new medium—the oscillations per second stay the same. However, light slows down in glass (due to interaction with atoms), and since $v = f\lambda$, a lower speed with constant frequency must mean a shorter wavelength. Think of it like a marching band entering mud: the beat (frequency) stays the same, but the soldiers move slower and bunch closer together (shorter spacing, or wavelength).