![]() ![]() Light travels at approximately 300,000 kilometers per second in a vacuum, which has a refractive index of 1.0, but it slows down to 225,000 kilometers per second in water (refractive index of 1.3 see Figure 2) and 200,000 kilometers per second in glass (refractive index of 1.5). When light traveling through the air enters a different medium, such as glass or water, the speed and wavelength of light are reduced (see Figure 2), although the frequency remains unaltered. How Fast Is the Speed of Light in Air and Water? Thus, after light has traveled twice a given distance, the intensity drops by a factor of four. ![]() However, the intensity of light (and other electromagnetic radiation) is inversely proportional to the square of the distance traveled. This well-established scientific fact is not a product of the Atomic Age or even the Renaissance, but was originally promoted by the ancient Greek scholar, Euclid, somewhere around 350 BC in his landmark treatise Optica. Light traveling in a uniform substance, or medium, propagates in a straight line at a relatively constant speed, unless it is refracted, reflected, diffracted, or perturbed in some other manner. Not truly a constant, but rather the maximum speed in a vacuum, the speed of light in km, which is almost 300,000 kilometers per second, can be manipulated by changing media or with quantum interference. ![]() The speed of light, which scientists have thoroughly examined, is now expressed as a constant value denoted in equations by the symbol c. ![]()
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