The speed of light in a vacuum, like that of the outer space, travels at 299,792,458 meters (186,282 miles) per second. In physics the speed of light is denoted as c, and is the greatest speed at which all matter and energy can travel in the universe. Light is measured by its wavelength and frequency. Wavelength multiplied by frequency is equal to the speed of light. The speed at which light travels through all forms of energy in the electromagnetic spectrum is the speed of light c. The speed of light in matter, such as water or glass, changes and is called the index of refraction.
Ether in space
For centuries, scientist knew how things, such as sound travels through air; however, they did not understand how sound, which is a wave was not able to travel through a vacuum, whereas light, which consists of waves could. Although, many physicists tried to find a medium for light to travel through space, they did not find any for many years. Scientists believed that space was not empty and that some kind of substance must exist there in order for light to travel. They called this substance ether. Experiments made later, proved that ether did not exist in space, and that light must have to be studied as a wave, although a wave of another kind of that of sound.
Discovering how light travels in space
In the 17th century, scientists thought that the speed of light was not finite and that it could travel instantaneously through space. One of the first experiments was made by Galileo, who tried to measure the speed of light from hilltop to hilltop, but he failed. It was necessary to use a faraway point of reference. During the 1670s, a Danish astronomer named Ole Roemer made observations of Jupiter´s Moon Io, discovering that light travels at a finite speed as opposed to instantaneously as it was previously thought. Later, James Clerk Maxwell studied the propagation of electromagnetic radiation though space, discovering how charged particles create a wave like motion similar to those created when you throw a rock in a pond of water.
Electromagnetic radiation describes the way in which light travels through space. The electromagnetic radiation includes all forms of energy in the electromagnetic spectrum, such as radio waves, infrared light, visible light, ultraviolet light, x-rays and gamma rays. All these forms of energy travel through space at the speed of light, and the energy is transmitted in the form of fluctuating electric and magnetic fields. Light is measured by its frequency expressed in Hertz and its wavelength, which are inversely related. Wavelength multiplied by frequency is equal to the wave´s velocity. In the electromagnetic spectrum, wavelength by frequency is always the speed of light c.
The speed of light in matter
The speed at which light travels through transparent materials, such as glass or water is less than the speed of light in a vacuum. The ratio between the speed of light and the speed of light in a material is called the index of refraction. Thus, c/v is equal to n. where c is the speed of light, v is the speed of light in a material and n is the index of refraction. The index of refraction of glass is approximately c/1.5= 200,000 km (126,000 miles) per second. The speed of light in air is typically 90 km less than the speed of light in a vacuum.
On Earth, light can be thought as moving instantaneously; however, this changes in outer space, where the speed of light can be quantified, thus, it takes the light of Alfa Centaury, our nearest star, 4.2 years to reach us here on Earth. The time it takes for light to circle the Earth is 140 milliseconds. Distance communication through space usually takes from minutes to hours, such as with distant space probes. According to grs.nasa.gov, light travels at a constant finite speed of 186,000miles/second, and it would take light one second to go around the Earth 7.5 times. A jet aircraft flying at the speed of 500 mph, would travel across the continental U.S. in four hours.