The speed of light travels at 299,792,458 metres per second, which is about 300,000 km per second. The speed of light is usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time. This speed is approximately 186,282 miles per second. It is the maximum speed at which all energy, matter and information in the universe can travel.
It is the speed of all mass-less particles and associated fields - including electromagnetic radiation such as light in vacuum, and it is predicted by the current theory to be the speed of gravity. The speed at which light propagates through transparent materials, such as glass or air, is less than c. For visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s. The speed of light through sodium chloride, otherwise known as common salt, is about 1.54.
In most practical cases, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light has noticeable effects. In communicating with distant space probes, it can take minutes to hours for the message to get from Earth to the spacecraft and back. The light we see from stars left them many years ago, allowing us to study the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. Finally, the speed of light can be used with time of flight measurements to measure large distances to high precision.
It is the speed of all mass-less particles and associated fields - including electromagnetic radiation such as light in vacuum, and it is predicted by the current theory to be the speed of gravity. The speed at which light propagates through transparent materials, such as glass or air, is less than c. For visible light the refractive index of glass is typically around 1.5, meaning that light in glass travels at c / 1.5 ≈ 200,000 km/s. The speed of light through sodium chloride, otherwise known as common salt, is about 1.54.
In most practical cases, light can be thought of as moving instantaneously, but for long distances and very sensitive measurements the finite speed of light has noticeable effects. In communicating with distant space probes, it can take minutes to hours for the message to get from Earth to the spacecraft and back. The light we see from stars left them many years ago, allowing us to study the history of the universe by looking at distant objects. The finite speed of light also limits the theoretical maximum speed of computers, since information must be sent within the computer from chip to chip. Finally, the speed of light can be used with time of flight measurements to measure large distances to high precision.
