When light transitions from one medium to another, such as from air to water or from air to glass, its velocity changes. This change in velocity is due to the difference in optical properties (such as refractive index) between the two mediums.
When light travels from a medium with a lower refractive index to a medium with a higher refractive index, such as from air to water, it slows down. This reduction in speed is quantified by the refractive index of the second medium. For example, the refractive index of water is approximately 1.33, meaning light travels roughly 1.33 times slower in water compared to its speed in a vacuum or air.
Conversely, when light transitions from a medium with a higher refractive index to a medium with a lower refractive index, such as from water to air, it speeds up. The refractive index of the first medium determines the degree of change in velocity.
It's important to note that the change in velocity occurs for all wavelengths of light, not just visible light. The phenomenon of light changing its speed when moving between different media is known as refraction. Refraction affects all wavelengths of the electromagnetic spectrum, from radio waves to gamma rays. The amount of bending and change in velocity depends on the specific refractive indices of the media involved and the wavelength of the light.
The change in velocity and bending of light due to refraction is fundamental to various optical phenomena, such as the formation of rainbows, the action of lenses, and the appearance of objects in water.