Light doesn’t always travel at the speed of light. The speed of light can change, depending on the type of materials it is passing through. Light will travel at the constant speed known as the speed of light if it travels in a vacuum or in a situation where there is no impediment to slowing the progress of that light. The slowest traveling light obtained in a controlled laboratory environment was 38 miles per hour (61 km / h), or 17,647,768 times slower than the speed of light. This phenomenon was created by directing a laser beam at cold sodium atoms, triggering a significant reduction in the speed of light.
More information on light travel:
Light traveling through glass or water loses speed when light photons come into contact with the other molecules that make up these substances. This activity causes photons to disperse, slowing forward motion.
The speed of light in vacuum is 186,282 miles per second (299,792.5 km / s). The speed of light in vacuum is usually described as speed c.
Materials that have a high level of refraction tend to slow down light considerably. In addition to water and glass, materials such as alcohol, whale oil, and fused quartz are considered optically dense media that have the ability to disperse photons of light and slow the speed of light.