# Speed of light

Nothing in the Universe is faster than the speed of light. What this special quantity is about and how you can measure the speed of light at all, you can find out here.

Look in any case still the Video on the subject of. In it all relevant contents are audiovisually prepared for you.

• Speed of light simply explained
• How fast is speed of light?
• speed of light measurement
• Speed of light formula
• Speed of light in electrodynamics

## speed of light simply explained

The Speed of light is probably one of the most important, basic constants in physics. It is exactly defined as, 299 792 458 meters per second (that is approximately 300 000 kilometers per second). This means that light travels 300,000 kilometers in one second. If you stand on the moon and shine a strong light source to the earth, the light needs 1.25 seconds before it is seen here.

In fact, this velocity value is exact, since by international agreement the definition of a meter states that a meter is equal to the length that light travels in Vacuum in" /> seconds.

Note, however, that this is really only the Speed of light in vacuum trades. If light penetrates a medium, even if it is only air, this speed is reduced.

The speed of light is defined exactly as 299 792 458 meters a second. Nothing moves faster than light.

## How fast is the speed of light?

It is hard to imagine how fast Speed of light is. The best way to visualize this is in relation to speeds that are more familiar to you. In the next table, let’s see how fast other things in our everyday life move compared to the speed of light speed of light are.

 Object speed in m/s (rounded) human being 10 Racing cars 100 sound waves 343 Supersonic aircraft 400 Light 300 000 000

However, we are dealing here with the vacuum speed of light, i.e. the speed of light in a vacuum (z.B. the universe). If light moves in a medium such as air, its speed is sometimes reduced considerably.

The speed of light is the maximum speed of all matter and information. On conventional paths, nothing is faster than the vacuum light velocity . This means that not only light but everything is subject to this speed limit. This includes for example electromagnetic radiation and Gravitational waves. Such Waves and Particle move with speed of light, independent of the speed and direction of its source. This also applies to a moving object. For example, if a train is moving with its lights on, the light will still move at the speed of light, no matter how fast the train is moving. Particles and matter whose Mass is not zero can change the Speed of light but never reach it. ### Speed of light in vacuum

In transparent media such as air or glass, light travels slower than the Vacuum speed of light. The same is true for electromagnetic waves in conductors. These also move slower than speed of light. This relation of Speed of light to the speed in the medium is called Refractive index .

### Speed of light c air

In air this Refractive index for visible light 1.0003. Therefore, light in air travels about 90 kilometers per second slower than in Vacuum.

\approx 299910" /> km/s

### Speed of light in water

In water the Refractive index about 1,3. The speed of the light reduces thus to approximately 230 769 kilometers per second.

\approx 230769" /> km/s

### speed of light in glass

In glass the Refractive index 1,5. If you calculate this as before you get a speed of about 200 000 kilometers per second.

\approx 200000" /> km/s

## Speed of light measurement

When you turn on the light at home, it seems as if the light immediately fills the room. However, if you look at it over very long distances and with better measuring equipment than your naked eye, the finite speed of light becomes clear.

There are many such experiments. However, an interesting one uses our moon as a target.

Imagine you put a mirror on the surface of the moon. Now you use a Laser to shine light from the earth onto this mirror and wait how long it takes to see the reflected light. Only after about 2.5 seconds you see the mirror flash up.
How fast" /> thus moves your Laser light?

You can calculate that. The moon is 384,400 kilometers away from the earth. This distance must be your Laser light twice to bridge the distance. Once to get from your home to the moon and then again to get from the moon back to you. For this distance the laser needs 2.5 seconds.

=\frac=\frac=307520 \, \frac" />

This is not quite the same as the actual value of about 300,000 kilometers per second, but it is very close. With more precise measuring instruments you can calculate the Speed of light more precise.

By the way, light needs even longer time from the sun to the earth. The light emitted by the sun takes 8 minutes and 20 seconds to reach us on earth.

## Speed of light formula

Now you have seen that the speed is given in meters per second but also in kilometers per second or kilometers per hour. With simple calculations you can convert the values to the corresponding units yourself.

To make it easier to calculate, we round up and say that the speed of light is 300 000 000 meters per second (300 000 000 m/s).

### speed of light in km/s

The conversion in kilometers per second is relatively simple. One kilometer has 1000 meters. You know that your light propagates with 300 000 000 meters per second.

=300000 \, km" />

That means in one second your light propagates 300 000 kilometers.

### Speed of light in km/h

Now you know how far your light travels in one second. Now you only have to extrapolate this to the hours. One hour has 60 minutes. One minute has 60 seconds.

One hour has 3600 seconds.
To calculate the speed in kilometers per hour, you have to expand with 3600. From the fraction calculation you know this surely. Your speed is a fraction" />.

= \frac=1080000000 \, \frac" />

Light moves in the Vacuum therefore with about 1.080.000.000 km/h (kilometers per hour).

## speed of light in electrodynamics

The classical physics describes light as a kind of electromagnetic wave. Furthermore describe the Maxwell’s equations the classical behavior electromagnetic waves. These predict Speed of light the electromagnetic waves in Vacuum with the distributed Capacity and Inductance of the Vacuum is related. These are respectively Permittivity of the vacuum and the Permeability expressed.

>" />

In modern Quantum physics describes the theory of Quantum electrodynamics (QED) describes the electromagnetic field. In this light is a fundamental excitation (also a Quantum called) of the electromagnetic field. This excitation takes the form of photons to. In the context of QED Photons massless particle. Therefore, these also move, according to the special theory of relativity, with speed of light by the Vacuum.