The speed of light in a vacuum, denoted by the symbol "c," is considered a fundamental constant of nature. Its value is approximately 299,792,458 meters per second (or about 186,282 miles per second).
The speed of light in a vacuum is determined through experimental measurements using various techniques. One of the most notable historical experiments is the Michelson-Morley experiment conducted in the late 19th century, which provided evidence for the constancy of the speed of light. Since then, numerous experiments have been conducted using different methods to measure the speed of light more accurately.
In modern times, one common technique involves using lasers and highly precise timing instruments. The basic idea is to measure the time it takes for light to travel a known distance. By dividing the distance traveled by the time taken, the speed of light can be calculated.
Regarding your second question, according to our current understanding of physics, the speed of light in a vacuum is considered constant and does not depend on the source or destination. This principle is a fundamental postulate of Einstein's theory of special relativity. According to this theory, the speed of light is the same for all observers regardless of their relative motion.
However, the speed of light can be affected when it passes through different materials or media, such as air, water, or glass. When light travels through a medium other than a vacuum, it can be slowed down due to interactions with atoms or molecules in that medium. This is why light appears to change speed when it passes through substances like water or glass. The speed of light in a specific medium is typically denoted as "v" and is slower than its speed in a vacuum (c).
So, while the speed of light is considered constant in a vacuum, it can vary when passing through different materials, but this variation is distinct from its speed as measured in a vacuum.