When light moves through a medium, such as glass or any other transparent material, the wavelength typically decreases while the velocity also decreases. This phenomenon is known as the slowing down or the reduction in speed of light in a medium.
According to Snell's law, which describes the behavior of light at the interface between two media, the velocity of light in a medium is inversely proportional to the refractive index of that medium. The refractive index is a measure of how much the speed of light decreases when it enters a particular medium compared to its speed in a vacuum.
In general, the refractive index of most materials is greater than 1, which means that the velocity of light is lower in those materials compared to its velocity in air or vacuum. As a result, the wavelength of light decreases as it passes through these materials.
However, there are a few exceptional cases where the wavelength and velocity of light can increase in a medium. One such case is anomalous dispersion, which occurs in certain materials and under specific conditions. Anomalous dispersion is characterized by a decrease in refractive index with increasing wavelength.
In these materials, as the wavelength of light increases, the refractive index decreases, and consequently, the velocity of light can increase. However, it's important to note that even in anomalous dispersion, the increase in wavelength and velocity is limited to a specific range of wavelengths, and it is not a common occurrence.
As for a medium that keeps the wavelength and velocity of light as close to air as possible, gases are the most suitable option. In gases, such as air or helium, the refractive index is very close to 1, meaning that the velocity of light in these mediums is nearly the same as in a vacuum. This results in minimal changes to the wavelength of light as it passes through gases, keeping it close to its original value in air.