When a wave propagates from one medium to another, such as from a less dense medium to a denser medium, its velocity can change. This phenomenon is governed by a principle known as Snell's law or the law of refraction.
According to Snell's law, the velocity of a wave decreases when it enters a denser medium. This change in velocity is primarily due to the change in the wave's direction caused by the change in the speed of propagation. When a wave encounters a change in medium, it interacts with the atoms or molecules of the new medium, leading to the wave being absorbed and re-emitted by the particles. This absorption and re-emission process causes a delay in the wave's propagation.
In a denser medium, the atoms or molecules are typically closer together compared to a less dense medium. This closer arrangement results in stronger intermolecular or interatomic forces, which tend to impede the wave's motion. Consequently, the wave has to overcome more interactions with the particles in the denser medium, leading to a slower overall propagation speed.
It's important to note that the change in velocity occurs because the frequency of the wave remains constant while its wavelength changes. According to the wave equation (v = λf), where v represents velocity, λ represents wavelength, and f represents frequency, if the velocity decreases, and the frequency remains constant, the wavelength must also decrease. This change in wavelength is responsible for the bending or refraction of the wave as it enters a denser medium.
In the context of optical density, which refers to the refractive index of a material, the decrease in wave velocity when entering a denser medium is a result of the change in refractive index. Refractive index is a measure of how much the speed of light is reduced when it enters a particular medium. Higher refractive index values indicate denser materials with slower wave velocities.