When light passes through a medium other than a vacuum, such as air, water, or glass, it can appear to slow down. This phenomenon is due to the interaction between light and the atoms or molecules of the medium.
In a vacuum, light travels at its maximum speed, known as the speed of light, which is approximately 299,792,458 meters per second in a vacuum (often rounded to 3.00 × 10^8 m/s for simplicity). This speed is a fundamental constant in physics denoted by the symbol 'c.'
In a medium, light encounters atoms or molecules that have charged particles (such as electrons) and positive nuclei. When an electromagnetic wave, such as light, interacts with these charged particles, it induces them to oscillate. These oscillations give rise to secondary waves that spread out in all directions.
The secondary waves interfere with each other, causing a delay in the overall propagation of the light wave. This delay is what we perceive as a reduction in the speed of light in the medium compared to its speed in a vacuum.
To understand this phenomenon more quantitatively, we introduce the concept of refractive index. The refractive index of a medium is a measure of how much slower light travels in that medium compared to a vacuum. It is denoted by the symbol 'n.' The refractive index is defined as the ratio of the speed of light in a vacuum (c) to the speed of light in the medium (v):
n = c/v
As a result, when light passes through a medium with a refractive index greater than 1, it appears to slow down because its velocity is reduced compared to its velocity in a vacuum.
The refractive index varies depending on the characteristics of the medium, such as its density and the interactions between the light and the atoms or molecules within it. Different substances have different refractive indices, which is why light travels at different speeds in materials like air, water, or glass.
It's important to note that the speed of light in a medium is not entirely determined by the speed of light in a vacuum. It depends on the properties of the medium and the interaction between light and matter within that medium.