For a long time, the founders of wave optics T. Jung and O. Fresnel knew that light waves are longitudinal, that is, they are similar to sound waves. At that time, light waves were perceived as elastic waves in the ether, which fill the entire space and penetrate into each body. It seemed that such waves could not be called transverse.
But nevertheless, a little more and more experimental evidence and facts were being gathered that could not be explained, assuming that the light waves were longitudinal. After all, transverse waves could exist exclusively in solids. But how can a body move in ether without resistance? The ether should not slow down the movement of bodies. Indeed, otherwise the law of inertia would not be fulfilled.
One simple and useful experiment with a tourmaline crystal may be considered. It is transparent and has a green color.
A tourmaline crystal has an axis of symmetry. This crystal is classified as uniaxial crystals. A rectangular tourmaline plate is taken, cut so that one of its faces is parallel to the axis of the crystal itself. If the beam of electric or sunlight is directed normally to this plate, then the rotation of the plate around it will not cause changes in the intensity of the light that passes through it. There is a feeling that the transmitted light in the tourmaline was partially absorbed and acquired a light green color. Nothing else happens. But this is a mistake. The wave of light acquires new properties.
They can be detected if a beam of light passes through the same second tourmaline crystal, which is parallel to the first. When the axes of the two crystals are in the same direction, nothing interesting happens either, only the light beam is weakened more and more due to absorption, passing through the second crystal. But when the second crystal rotates, if the first one is left motionless, an interesting phenomenon called “damping of light” will be discovered. In the process of increasing the angle between these two axes, the saturation of the transmitted light beam decreases. When two axes are perpendicular to one another, light cannot pass at all. It will be completely absorbed by the second crystal. How is this explained?
Transverse light waves
From the description of the facts shown earlier, it follows:
1. Firstly, the light wave that comes from the light source is absolutely symmetrical with respect to the direction in which propagation occurs. During the revolution of a given crystal around a passing ray of light, during the first experiment, its intensity did not change.
2. Secondly, the wave emerging from the first crystal will not have axial symmetry. The intensity of the passing light through another crystal depends on its rotation.
Longitudinal waves differ in complete symmetry with respect to the direction of propagation. Oscillations of longitudinal waves occur along this direction, this oscillation is the axis of symmetry of the wave. That is why it is not possible to explain the experiment with the rotation of the second crystal, considering the wave of light to be longitudinal: these are transverse waves.
The experience can be fully explained by making two assumptions:
Assumption number one relates directly to light: light waves are transverse waves. But in a beam of light waves incident from a light source, there are vibrations of various directions that are perpendicular to the direction in which such a wave propagates. In this case, considering this assumption, we can conclude that the wave of light has axial symmetry, while at the same time being transverse. For example, waves on the water surface do not have similar symmetry, because the oscillations of water particles occur exclusively in the vertical plane.
Waves of light with oscillations in various directions that are perpendicular to the directions of propagation are called natural. This name is justified, because under standard conditions, different light sources create just such waves. This assumption is explained by the results of the first experiment. The rotation of the tourmaline crystal does not change the saturation of the transmitted light beam, because this incident wave has axial symmetry, even though it is a transverse wave.
The second assumption relates to the crystal itself. Tourmaline has the ability to transmit light waves with vibrations that occur in a certain plane. This light is called polarized (or plane-polarized). It differs from the natural, unpolarized.
This assumption is explained by the second experiment. Plane-polarized light (wave) emerges from the first tourmaline crystal. When crystals are crossed at an angle of ninety degrees, the wave cannot pass through the second of them. If the cross angle is different, then oscillations will take place , the amplitude of which will be equal to the projection of the amplitude of the wave passing through the first plate in the direction of the second axis. This is precisely the proof of the theory that light waves are transverse waves.