All forms of electromagnetic waves, including light waves, travel at the same exact speed in a vacuum, approximately 299,792,458 meters per second or 186,282 miles per second.
However, when light waves enter a material such as glass, water, or air, their speed is affected due to the interactions with the atoms and molecules in the medium.
The change in speed is often referred to as refraction and causes the light waves to bend or change direction. The refractive index of a material, which is the ratio of the speed of light in a vacuum to the speed of light in that medium, determines how much the wave's speed changes.
In general, the speed of light decreases when it travels from a vacuum to a denser material. For example, when light enters water, its speed reduces by about 25%. As a result, the wavelength of light also changes when it enters a different material while maintaining the same frequency.
