The speed of light and sound in a medium is determined by the properties of that medium, such as its density, elasticity, and other physical characteristics. When the density of a medium increases, the speed of light and sound generally increases as well.
To understand this, let's consider light first. In a transparent medium, such as air, water, or glass, light travels as electromagnetic waves. When light enters a denser medium, such as from air to water, it interacts with the atoms or molecules of the medium. The increased density means that there are more atoms or molecules per unit volume, and this increases the frequency of collisions between light and the medium. These collisions cause the light to scatter and interact with the medium's particles, resulting in a slower effective speed.
However, despite these interactions, light still propagates at a high speed in a dense medium compared to other particles, because the electromagnetic force responsible for light propagation is extremely strong. In some cases, such as when light enters a denser medium from a less dense medium, the speed of light can increase due to certain properties of the medium, such as its refractive index. This effect is responsible for phenomena like refraction.
Regarding sound, it also travels in waves, but as mechanical vibrations of particles in a medium, such as air or water. When the density of the medium increases, there are more particles per unit volume, and this results in more collisions and interactions between the sound wave and the particles. These interactions can enhance the speed of sound propagation because the particles transmit the vibrations more effectively.
It's important to note that the relationship between the density of a medium and the speed of light or sound is not always straightforward and can depend on various factors. Different materials have different properties, and other factors like temperature and pressure can also influence the speed of light and sound in a medium. The behavior of light and sound in different media is a complex field of study, known as optics and acoustics, respectively.