Speed of Light

Not only humans, cars, planes, etc. are able to move with a certain speed, but light has a speed as well. In this case, it is called speed of propagation. That is, when you switch on a lamp, the light is not everywhere instantly, but reaches the most distant corner a little later, similar to a plane starting in Vienna and reaching London approximately an hour later. It is not perceptible for us that the light reaches the remotest corner a little later, though, as the speed of propagation of light is very fast. From switching on the lamp until the moment the light reaches the corner, only very little time passes.
The velocity of light (it is often indicated with c) is the fastest speed we know. Nothing can move faster than light. The speed of light is a constant of nature, so to say. In the vacuum (that is, in space) it is about 300 000 000 m/s, which is equivalent to more than 1 000 000 000 km/h!! This is such a great number, that it is almost impossible to imagine it.

Is Light Always Equally Fast?

The speed of light is not the same everywhere. In the air, it is slightly slower, but still almost as fast as in vacuum. In water, it is already noticeably slower. Here is a short overview, in which materials light spreads with which speed:

Where Is the Sun When We See It?

The distance between the sun and the earth is approx. 150 000 000 km. The velocity of propagation of light in space is known as well. With these two pieces of information, the time the light needs to cover the distance from the sun to the earth can be calculated. The result is about 8 minutes and 20 seconds. So, if we believe that we see the sun at a certain spot, it isn’t there any longer in reality, but already a bit further. We see the sun where it was 8 min 20 sec ago. Therefore, we also see sunrise and sunset with a delay of 8 min 20 sec.

Always As Speedily As Possible!

An important property of light is that it always tries to travel the fastest way between to objects (this is not necessarily the shortest way!).

When you watch a fish in water, light moves along the following way (continuous black line):

We see the fish along the broken line, so a little raised. This is the reason, why our legs look a little shorter, when we are standing in the water: The feet - like the fish - appear a little raised. The shortest distance would be along the red line, but this is not the fastest way, so light does not move along the red line.