The Speed of Light (c) in a vacuum is 299,792,458.0 meters per second (m/s). This constant is provided in meters per second. However, this can be automatically converted to other velocity units via the pull-down menu.
The speed of light (symbol: c) is sometimes known as "light speed". Theoretically nothing can travel at a speed exceeding the speed of light. The speed of light is often used in aerospace and astronomy since space is nearly a perfect vacuum. The distances to very far away celestial objects such as stars and galaxies are often given in light years. A light year is the distance that something traveling at the speed would go in one year. On a lesser scale, there are light-seconds, light-minutes, light-hours and light days in the same vein.
Relationship between Wavelength and Frequency
The speed of light is the speed at which all electromagnetic waves travel in a vacuum and serves as the linear constant in the relationship between electromagnetic wavelength and frequency.
See electromagnetic wavelength and frequency relationships in the following:
Relativistic Energy and Momentum
See theto see how as the velocity, v, of a mass approaches the speed of light, c, the denominator approaches 0,and thus the equation at v = c is undefined. This is the basis for the premise that nothing can exceed the speed of light.
Use in Defining Astronomical Scale of Distance
Since the speed of light defines the upper bound of travel speed, travel times to distant objects in the universe are generally measured in terms of the speed of light. We describe these distances on astronomical scales in terms of light years of distance. A light year is the distance light travels in a vacuum in the span of a year.
The average time it takes light from our Sun to reach the Earth is approximately 8 minutes and 17 seconds.
The speed at which light propagates through, such as glass or air, is less than c. The ratio between c and the speed v at which light travels in a material is called the n of the material (n = c / v). For example, has a of 1.000293, so the speed of light in air is 299705 km/s or about 88 km/s slower than c. This refractive index in turn defines how light bends as it passes through the Earth's atmosphere.
Atmospheric refraction is the deviation ofor other from a straight line as it passes through the due to the variation in as a function of . This refraction is due to the velocity of light through decreasing (the increases) with increased density.
Atmospheric refraction causesto appear higher in the sky than they are in reality. It affects not only lightrays but all electromagnetic radiation, although in varying degrees (see ). For example in visible light, blue is more affected than red. This may cause astronomical objects to be spread out into a spectrum in high-resolution images.
The change in the speed of light is also what causes the rainbow effect of sunlight passing through a prism. As sunlight, which contains the entire spectrum of visible light, passes through the clear martial of a prism, the light changes velocity and the angle it passes through the medium. Since the different parts of the spectrum have different wavelengths, their path will be affected differently and the exiting light from the prism will have the visible spectrum spread noticeably.
This constant, the speed of light, c, is specified as an exact number
Since 1983 the meter, the standard SI unit of length, has been defined by an international agreement to be the distance traveled by light in vacuum during exactly 1/299,792,458 of a second. This makes the speed of light exactly 299,792.458 km/s.
The speed of light previous to its definition in 1983 had been measured by bouncing lasers off a mirror placed on the moon and measuring the round trip travel time of the light.
Veritasium video on
 Speed of Light
 Atmospheric refraction
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