Light travels at around 300,000 kilometers per second in a vacuum (refractive index of 1.0), but slows 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). Thus, light travels about half as fast in water and glass than it does in vacuum.
The reason for this slowdown is that when light passes from one medium with a different refractive index to another, some of it is reflected and some of it is transmitted. The amount of reflection and transmission depends on how closely the two media match in refractive index. For example, if you were to fill a glass with water and throw a rock into it, most of the rock would pass straight through the water without being affected by it. However, about one out of every ten rocks would be slowed down enough by the water to fall below a certain speed and be stopped completely. This is because the rock and the water are not matched in refractive index - the rock is made of solid material while the water is liquid-and thus, some of the rock's energy is lost when it enters the water.
Reflection and transmission of light are important factors in light beams passing from one medium to another. For example, when light enters water, some of it is reflected back toward the source and some of it spreads out in all directions like a wave.
The speed of light is determined by the medium through which it travels. The speed in empty space is 186,000 (1.86 x 10 5) miles per second. It's about the same in the air. It slows down to around 140,000 (1.4 x 105) miles per second in water. This is why radio signals travel faster in water than in free space.
You need a device called an "electrometer" to measure the speed of light in liquids. An electrometer uses two electrodes and a circuit to measure the voltage between them. When you put electric current into the circuit, the electrodes become polarized. This means that they lose some of their original charge and become positive and negative instead. If there is no voltage between them, then the charge on each electrode is equal and there is no net charge on the entire system. But if there is a voltage between them, then the electrode with more charge will be pushed away from the other electrode and we can say that there is a difference in charge between them. The amount of charge that has been moved is measured by a voltmeter. A voltmeter always shows the magnitude of voltage, so even if one electrode is positive and another is negative, the reading on the voltmeter will be zero.
In our experiment, one electrode will go into the liquid while the other stays out of it. This means that only half of the electrometer will be immersed in the liquid while the other half remains dry.
Light travels at a speed of 226 million m/s, or 205,600 mph, in water. Light travels at a speed of 200 million m/s, or 182,300 mph, in glass. Because light travels so quickly, measuring the difference in its speed in various materials is challenging. The best way to do this is with a standard laboratory experiment called a "reflection test." In this experiment, one end of a beam of light is placed in contact with the material being tested while the other end is shielded from it.
The time it takes for the light to travel the length of the beam of light is very slightly longer than it would be in clean air because some of the light is reflected back toward the source by particles in the liquid. Using this method, light speeds through water at about 1% of the speed it does in free-flowing air. Through most common solvents it is less than 1%.
Liquids also absorb light, which means that they too will slow down light over time. But since liquids evaporate or burn off, this effect is not as great as it is in solid materials. Light can pass through liquids as long as it doesn't interact with any atoms inside them; otherwise, it will be slowed down like everything else.
The speed of light is always the same in every medium, which means that it shouldn't matter how you move your source or detector around a sample of liquid.
In a vacuum, the speed of light is 299,792.458 km/s—just shy of a nice round 300,000 km/s figure. That's a lot of wind. The Sun is 150 million kilometers away from Earth, and light travels that distance in approximately eight minutes and twenty seconds. If the Sun were instead located exactly halfway between Earth and the Moon (which is closer than it seems) then sunlight would take just under nine minutes to reach them.
Therefore, if you were standing on the Moon and watched a sunrise over Earth, you would see the sun rise twice: once when it was actually daylight here on Earth, and again when we saw its rays last from our lunar vantage point. As soon as the sun reaches its highest point in the sky here on Earth, it starts to get darker around us, but only until about an hour after sunrise when the day finally gets going.
Since light travels at a constant speed, this means that the farther away something is, the later it will reach its destination. So even though it's daytime everywhere else, there will be no sunlight reaching the Moon or anywhere else outside our planet, because the Moon is too far away from Earth.
However, objects with mass can affect the rate at which information travels through space. For example, radio signals travel at the same speed in a vacuum, but if there are clouds or mountains in their path they will be slowed down.
The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and nothing can go faster than light. Light speed is about mph in miles per hour. The distance that light travels in one second is called its wavelength, and since the diameter of the sun is about 864,000 miles we know that light has traveled at least 864,000 miles in just over 10 minutes.
This means that if you were to start out on the surface of the sun and ride your bike home, it would take you nearly eight days to cover the distance between the earth and the sun! But what happens if we travel much faster than light speed? Well, according to Einstein's theory of relativity, everything around us will be red shifted - the wavelengths of all the colors of the spectrum will be reduced. For example, when astronauts go into orbit around Earth, they see all the colors of the spectrum, but because they're moving so fast, everything around them appears red-shifted into the ultraviolet part of the spectrum.
Now, what happens if we go even faster than that? Well, according to theoretical physics, if you could travel close enough to the speed of light you would see some strange effects appear. First of all, everything around you would be blue shifted - the wavelengths of all the colors of the spectrum would be increased rather than decreased.