Light, wave or particle?

Isaac Newton maintains that light consists of grains, while the Dutchman Christian Huygens maintains that it is made of waves. The corpuscles are punctate entities in the sense that they are small material points, located in a very limited region of space, like grains of sand, whose size would tend to zero. They describe clear trajectories in space, after which their position and speed at any moment are clearly determined. On the contrary, the waves are not precisely localized; they occupy, if not all space, at least a certain spatial extent.

What distinguishes waves from blood cells?

Another thing distinguishes waves from blood cells. Waves transport nothing, while blood cells transport themselves. To be better understood, this ambiguous phrase deserves an enlightening illustration: if we stretch a rope and then wave one of its ends, we generate a wave which propagates along the rope without the rope leaving our hand to run after the rope. whereas if we throw a material body into the hand, for example a stone, there always comes a time when it ceases to be in contact with the hand to move alone in space.

Think of the waves on a pond

Ultimate difference, which will be decisive for the rest of the story: the waves are able to “overlay”, that is, to add to each other. Consider the waves we can create on a pond. If we wave a stick in the water at a certain place, we create a system of concentric waves from it. If we wave the stick in another place, we create another wave system centered in that other place. If we simultaneously wave a stick in each of these two places, we evoke a more complicated wave system, which is nothing more than the superposition of the previous two wave systems. It is obtained by adding the amplitude of the first wave system to each point in the pond to the second. This property of being able to add together in this way keeps the Wave Cartel at it exclusively: two blood cells would be quite unable to do the same.

Light according to Newton: a kind of grapeshot consisting of blood cells moving in empty space

All these differences mean that there does not seem to be the slightest connection between waves and blood cells or, if you prefer, between the delocalized motion of a wave on the surface of the ocean and a jet of stone in the air in a very clear trajectory. For every physical phenomenon, therefore, the question arises: does it belong to the category of waves or to the category of corpuscles?

It is about light that this question has reached the most dramatic intensity because the answer to it has fluctuated throughout the history of physics. At the end of the 17th century, Isaac Newton believed that light was a kind of grapeshot consisting of blood cells moving in empty space. But at the beginning of the 19th century, after experiments carried out and interpreted by Thomas Young and Augustin Fresnel, the idea gradually gained ground, already defended by Huygens, that light is not a particle phenomenon, but a wave phenomenon. . It is actually able to produce interference, which circles in water when they cross each other. In short, light added to light can produce more light when it adds constructively to itself, or conversely generate darkness when it adds destructively.

In the second half of the 19th century, it was concluded that light was actually a vibration circulating in space, a propagation of electromagnetic waves, each of these waves being characterized by its frequency, that is, the number of its oscillations in a second. This conclusion did not seem to be open to dispute as the facts required it. And still…

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