The speed of light in a vacuum is exactly 299,792,458 m/s. In 1983 physicists agreed to define a metre using the speed of light, i.e. a metre is defined as the distance traveled by a ray of light in a vacuum in 1/299,792,458s. The speed of light reduces when it passes through matter. It is hardly slowed down by air, but the speed of light through glass is reduced to around 200 000 km/s. Less dense …showed first 75 words of 303 total…

…showed last 75 words of 303 total…always travels at the same speed relative to some observer, no matter what the relative motion of the observer. So, "light emitted from a moving airplane does not travel with the speed of light plus the speed of the airplane, it travels with the "speed of light", no matter what the speed of the airplane! In a vacuum, light always travels at a speed of 299,792,458 meters per second, no matter how its speed is measured."