speed of light, speed at which light waves propagate through different materials. In particular, the value for the speed of light in a vacuum is now defined as exactly 299,792,458 metres per second. The speed of light is the rate at which light travels. The speed of light in a vacuum is a constant value that is denoted by the letter c and is defined as exactly 299,792,458 meters per second. The speed of light in vacuum, often called simply the speed of light and commonly denoted c, is a universal physical constant exactly equal to 299 792 458 m⋅s−1. [2] From the earliest experiments to the latest discoveries in cosmology and quantum mechanics, the speed of light remains the ultimate speed limit—an elegant, immutable boundary that continues to inspire wonder, challenge our intellect, and shape the very nature of reality. The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. What is the Speed of Light in Miles Per Hour? Definition: The speed of light in a vacuum is a fundamental physical constant important in many areas of physics, exactly 670,616,629.3844 miles per hour. The speed of light, commonly denoted as c, is a fundamental constant in physics representing the maximum speed at which all energy, matter, and information in the universe can travel. Light travels at a constant, finite speed of 186,000 mi/sec. A traveler, moving at the speed of light, would circum-navigate the equator approximately 7.5 times in one second. The speed of light is important because it’s about way more than, well, the speed of light. In the early 1900’s Einstein realized just how special this speed is. The speed of light — c = 299,792,458 m/s exactly — is the most fundamental constant in physics. It is the speed at which all electromagnetic radiation travels through a vacuum.