الاشعة فوق البنفسجية - الاشعة السينية .. الفيزياء

Ultraviolet rays - X-rays .. Physics

Ultraviolet rays

The human eye does not respond to light waves whose length is shorter than the wavelength of the end of the violet color. It is known that the ultraviolet section is located directly after the visible rays section in the electromagnetic spectrum and that its wavelength ranges between 10 and 380 nanometers.

Although the ultraviolet rays emitted by the sun do not exceed 50% of the total radiation emitted by it, the energy produced by it is characterized by being large due to its high frequency. It is also known that the interaction of ultraviolet rays with atoms and molecules leads to the displacement (ionization) of the latter - that is, to the liberation of electrons in them, leaving behind ions loaded with positive charges, and ultraviolet rays are one of the types of stray rays, in addition to other types, which are X-rays and gamma rays, which have displacement energies that greatly exceed the energy of the former.

Ultraviolet rays from the sun cause the oxygen molecules in the ionosphere to be scattered and create ozone (O3) which forms in the upper part of the atmosphere an absorbent layer for these rays that only allows a small portion of them to penetrate the Earth's surface. The remaining portion of these rays, whose wavelength exceeds 200 nanometers, is able to penetrate the Earth's atmosphere and provide the human body with many benefits. Some reactions, such as those that produce vitamin D, do not occur except in the presence of these rays. There is a narrow range of them with a wavelength of about 300 nanometers, whose mission is to produce melanin in the skin and give the skin its bronze-brown color.

X-rays

The X-ray section of the electromagnetic spectrum lies between ultraviolet rays and gamma rays, and its wavelength ranges from approximately 0.1 to 10 nanometers. These rays were discovered by William Rantgen in 1895, and they are generated by the collision of some fast electrons with atoms of heavy elements, a collision that results in an exchange between the energy of the incoming electrons and the energy of the electrons present in the inner layers of the target atom, so the latter jumps from one orbit to another, and when it returns to its original state, X-rays are emitted from it.

The most recent X-ray tube is the vacuum tube that contains a tungsten filament and a target of the same metal as well. In it, an electric current heats the filament until it glows and produces a vapor of electrons that are accelerated by a difference in electrical potential applied between the cathode on one hand, and the target that plays the role of the anode on the other hand. This voltage ranges between kilovolts and megavolts, and can control the energy of the X-rays, which is equal to the product of this voltage G and the electron charge E. If this energy can reach the target atom in its entirety, the frequency of the emitted rays becomes equal to G h/e.

As for the intensity of the rays, it is controlled by the current passing through the filament and the type of material that makes up the target itself.

There is a phenomenon that accompanies the wave motion of both light and X-rays, which is the phenomenon of diffraction. If the rays pass through a narrow slit, the edges of the slit become two coherent sources (in phase and frequency) of the rays, and as a result of the difference in the paths of the two waves, a specific diffraction pattern similar to the known interference pattern is formed, and the slit must be much smaller than the wavelength of the rays used.
It has become known, since the beginning of this century, that the distances separating the atoms are approximately equal to 10 nanometers in crystalline bodies, and that these distances are precisely what is required to obtain the diffraction phenomenon in X-rays.

If X-rays fall at an angle of A on a series of atomic planes separated by a distance F, the rays are scattered in different directions, and in each plane separately, and they do not reach their maximum intensity unless they are directed at an angle A as well. If the distance difference (2f sin) between two waves scattered from two adjacent planes is equal to an even number of wavelengths, a bright spot is generated in the diffraction pattern, but if the distance difference is equal to an odd number, the spot is dark. It is known that diffraction in X-rays is used to determine the atomic shapes of crystals based on studying the diffraction pattern itself. Crystallography has developed by means of these rays and has become capable of determining the shapes of crystals that make up some complex biochemical molecules such as blood and insulin.

(Production of diffraction pattern by X-ray beam)
(Diffraction of X-rays in crystal)