Light a a wave

Speaking of a wave it is essential to make an effort to mentions some basic characteristics of it, in order to comprehend the difficulties that early scientists faced.

The American Heritage Dictionary describes a wave as: a disturbance or a vacillation, which is propagated by a particle to another (inside a material or in the vacuum).

Consequently, if we throw a stone in the water of a lake, we will see a series of cycles, which will open. These cycles are transmitted with a certain speed. A bigger stone will cause still more intense undulations and so on. The above characteristics are known with specific terms, which are:

Frequencywhich shows the number of the tops of waves that will pass from some point in a specific time
Wavelengthwhich shows the distance between two tops
Amplitudewhich is the distance between the top and the bottom of a wave.

wavelength Now we can change our view on the subject. A gossip "travels" from Athens and very soon reaches Thessalonici, even if nobody has travelled from one city to the other. In this case we have to do with two movements: The one of the gossip that is propagated from Athens and the second one of the people, who, even though they do not travel between the two cities, they move inside their city and they propagate the news. We have just imported two new concepts, the wave velocity and the phase velocity. The wave velocity shows how fast the entire wave moves, while the phase velocity shows the internal movement of waves in their passage. An interesting difference between these velocities is that wave velocity does not transport energy, whereas the phase velocity does. As a result, it is possible to notice a phase velocity greater than the speed of light, because the speed of light concerns the transmitted energy.

Thus, when the previous stone reaches the water the waves are propagated in cycles, which unfold always more, but there exists a difference: The movement of the wave is different from the movement of the molecules of water. The molecules simply go up and down.

Huygens The wave theory could explain something that the particle theory could not: When light meets an oblique angle, it creates a proportionally oblique shade. If light was succession of particles, how could happen? However at the 18th century almost nobody could back the wave theory. Only C. Huygens, who had the courage to express his own ideas. The Huygens's Principle accepts that every point on a wave front of light may be considered to be the source of secondary waves that radiate from their centres with the same frequency, velocity, and wavelength as their parent. The principle explains what happens when light or other waves encounter a barrier or enter another medium. But it was not enough. The fame of Newton was enough to terminate a thought like this.

We see that waves can be more confusing than particles. A particle is characterized by its mass and by other quantities that can be measured (e.g. its charge). The daily experience certifies that particles and waves behave differently. For example: two people, who stand at the opposite sides of a wall, cannot throw a ball (body) to each other, because the ball will strike the wall. On the contrary they cannot speak to each other, because the voice (wave) will pass through the wall. But, if light is a wave, why cannot pass through the wall? This epicheirema could not be surpassed, thus the wave theory could not be justified.