The speed of light in a medium is determined by its refractive index. The refractive index is a measure of how much slower light travels in that medium compared to its speed in a vacuum. Different colors of light have different wavelengths, and their interaction with a medium can vary.
In general, the refractive index of a medium tends to be higher for shorter wavelengths (bluer light) and lower for longer wavelengths (redder light). This phenomenon is known as dispersion. Therefore, blue light tends to travel slower in a denser medium compared to red light.
To illustrate this, you can observe the phenomenon of chromatic dispersion in a glass prism. When white light passes through a prism, it refracts (bends) at different angles depending on its wavelength. This separation of colors is due to the varying refractive indices for different wavelengths of light, with shorter wavelengths (blue/violet) bending more than longer wavelengths (red).
So, in summary, blue light tends to travel slower in a denser medium compared to red light, as shorter wavelengths experience higher refractive indices and are subject to greater bending or slowing down.