Speed of light (c) - Unit Information & Conversion

Definition

The speed of light in vacuum, commonly denoted by 'c', is a universal physical constant important in many areas of physics. Its exact value is defined as 299,792,458 meters per second.

History

The concept that light travels at a finite speed has been explored since antiquity. Early quantitative estimates were made by Ole Rømer in 1676. Increasingly accurate measurements were made over centuries, culminating in the 1983 definition by the General Conference on Weights and Measures, which fixed the speed of light to its current value, thereby defining the meter based on 'c' and the second.

Common Uses

• Physics: Fundamental constant in Einstein's theory of relativity (E=mc²), electromagnetism, and cosmology.
• Astronomy: Measuring vast cosmic distances (light-years).
• Telecommunications: Determining signal propagation speed in fiber optics and wireless communication.
• GPS: Accurate timing based on signal travel time from satellites.

Unit FAQs

Is the speed of light always the same?

The speed of light in vacuum (c) is constant and independent of the observer's motion or the source's motion. However, light travels slower when passing through materials like water or glass. The ratio by which it slows down is the material's refractive index.

Can anything travel faster than the speed of light?

According to the theory of special relativity, no object with mass can be accelerated to the speed of light, and no information can travel faster than 'c' in a vacuum.

How is the speed of light related to the meter?

Since 1983, the meter is defined as the distance light travels in vacuum during a time interval of 1/299,792,458 of a second. This definition fixes the speed of light exactly.