# Measuring lightspeed

1. Dec 3, 2009

### lneilson

Hi

I was reading about the measurement of light years and was suprised to see it is the speed at which light goes through a vaacum. Is that because of the vaacum of space or does light have mass?

thanks

lneilson

2. Dec 3, 2009

### Neo_Anderson

Light is massless, yet has momentum.
The velocity of light was actually determined on the chalkboard. What do I mean by that? Well, lightspeed was determined by James Maxwell in 1856 after he divided his equation for magnetism by his equation for electricity. After Maxwell divided these two equations, he obtained a constant: 300,000/sec, which is precisely the velocity of light in vacuo.

Experiments to verify this constant have been performed to a very high degree of accuracy.

3. Dec 3, 2009

### DrGreg

Well, actually 299,792.458 km/s is precisely the value.

4. Dec 3, 2009

### arunma

As Neo mentioned, light does not have mass (but it can be bent by a gravitational field!). The reason we talk about a vacuum speed of light and a speed of light in a medium is because light interacts with the medium that it passes through, and this retards its speed. Light rays are actually oscillating electric and magnetic fields. When an electric field passes through matter, it can polarize the material, which will have an effect on the field. Likewise, magnetic fields can magnetize materials, which will also have an effect on the magnetic field. Polarization and magnetization affect light rays, and change its wave speed.

Another way to explain this effect is to think about light as a particle. When photons (i.e. the fundamental quanta of light) pass through matter, they can be absorbed by atomic electrons and then reradiated. This is a quantum effect caused by the fact that atomic electrons have discretized energy levels. The process of absorption and reradiation introduces a delay, which retards the speed of light in matter.

The speed of light in matter is a property of the material through which the light is passing. The vacuum speed of light, however, is a fundamental property of the universe. In the nineteenth century, physicists did experiments on electric and magnetic fields. They discovered how electric charges interact, as well as how magnetic dipoles interact. Because light is an electromagnetic wave, the information about electric charges and magnetic dipoles can be used to deduce the speed of light, without ever calculating the speed of a light ray. The speed of light was also directly measured by using an apparatus called a Fizeau Wheel (you can Google to get the details of this experiment). Remarkably, the two methods of calculating the speed of light agree, which demonstrates that light is indeed an electromagnetic wave.

5. Dec 4, 2009

### lneilson

they really gave me something to think about

lneilson