Transverse waves

For a long time the ancestors 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 can not be called transverse.

But still more and more experimental evidence and facts were accumulated, which could not be explained, assuming that the light waves are longitudinal waves. After all transverse waves could exist only in solids. But how can a body move in a solid without resistance? Ether does not have to slow down the movement of bodies. Otherwise, the law of inertia would not be fulfilled.

One simple and useful experiment with a tourmaline crystal can be considered. It is transparent and has a green color.

The tourmaline crystal has an axis of symmetry. This crystal is regarded as uniaxial crystals. Take a rectangular plate tourmaline, cut out so that one of its face was parallel to the axis of the crystal itself. If a beam of electric or sunlight is directed normally to this plate, then the rotation of the plate around it will not cause a change in the intensity of the light that passes through it. There is a feeling that the passing light in the tourmaline was partially absorbed and acquired a light green color. Nothing else happens. But this is erroneous. A wave of light acquires new properties.

They can be detected if a beam of light passesthrough the same second crystal of tourmaline, which is parallel to the first. With the same direction of the axes of the two crystals, nothing curious happens either, only the light beam is increasingly weakened by absorption, passing through the second crystal. But with the rotation of the second crystal, if the first one is left motionless, an interesting phenomenon called "light damping" will be found. In the process of increasing the angle between two given axes, the saturation of the transmitted beam decreases. When the two axes are perpendicular to each other, the light can not 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. First, the light wave that comes from the light source is absolutely symmetrical with respect to the direction through which the propagation takes place. When this crystal is rotated around the passing beam of light, the intensity of the first experiment does not change.

2. Secondly, a wave emerging from the first crystal will not have axial symmetry. The intensity of transmitted light through another crystal depends on its rotation.

Longitudinal waves differ in complete symmetryregarding the direction of propagation. Oscillations of longitudinal waves occur along this direction, this oscillation and is the axis of symmetry of the wave. That is why to explain the experience with the rotation of the second crystal, considering the wave of light longitudinal, it is not possible: these are transverse waves.

You can fully explain the experience, making two assumptions:

The assumption number one concernsdirectly to light: light waves - transverse waves. But in the beam of light waves incident from the light source, oscillations of various directions are present, which are perpendicular to the direction along which this wave propagates. In this case, considering such an assumption, we can conclude that the wave of light has an axial symmetry, at the same time being transverse. For example, waves on a water surface have no such symmetry, because the vibrations of water particles occur exclusively in the vertical plane.

Waves of light with fluctuations in variousdirections 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 beam of light, because this incident wave has an axial symmetry, even though it is a transverse wave.

The second assumption relates to the crystal itself. Tourmaline has the ability to pass waves of light with fluctuations that occur in a certain plane. This light is called polarized (or plane-polarized). It differs from the natural, unpolarized.

This assumption is due to the second experience. Flat-polarized light (wave) emerges from the first crystal of tourmaline. When crossing crystals at an angle of ninety degrees, the wave can not pass through the second of them. If the angle of the cross is different, then the oscillations will pass, 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 the proof of the theory that light waves are transverse waves.