The speed of light in a vacuum is approximately 299,792 kilometers per second (about 186,282 miles per second). This speed is considered a fundamental constant of nature and is denoted by the letter "c" in scientific equations. It plays a crucial role in the theory of relativity, which was developed by Albert Einstein. Light travels at different speeds in various materials, such as air, water, and glass, due to interactions with the atoms in those substances. This change in speed causes phenomena like refraction, where light bends as it passes from one medium to another.