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Electromagnetic Radiation

First of all we should make clear the fact that visible light is nothing but a very small portion of the entire electromagnetic spectrum. The electromagnetic spectrum includes some regions important for our everyday life such as the gamma rays, the X-rays, ultraviolet light, infrared radiation, microwaves etc. To measure the wavelength we use the term "nanometer" which represents 0.000000001 meter. As we mentioned in the previous chapter a high wavelength corresponds to a low frequency and visa versa.

Let us have a look at specific regions of the electromagnetic spectrum.

Radio waves

The usability of radio waves is extend. We use them for wireless transmission of sound messages, for communication. The AM (amplitude modulation) and FM (frequency modulation) involves radio waves. Some characteristic numbers about this part of the electromagnetic spectrum follow. The width is about 10,000 Hz for telephone, 20,000 Hz for high-fidelity sound, and five megahertz for high-definition television. The lower frequency limit for radio waves is near 10,000 Hz.


The microwave region extends from 1,000 to 300,000 MHz (or 30-centimetre to one-millimetre wavelengths). Microwaves make the communication between earth and space based stations and satellites feasible. As a result we use microwaves in meteorology, to measure distances in space and to research the properties of the matter. We also use them in communications (TV, FAX and telephone).Spectrum

Microwaves can be absorbed by water and fat in food. At the same time this radiations is not absorbed by glass, ceramics or metal foils. Scientists took advantage of it in order to manufacture microwave ovens. Microwave ovens can cook or heat food in very shorts periods of time.

Although microwaves are invisible to us we can feel the heat. On the other hand, exposure to microwaves is dangerous mainly when high densities of microwave radiation are involved. They can cause burns, cataracts, damage to the nervous system, and sterility.

Infrared radiation

Approaching the visible region of electromegnetic spectrum we meet the infrared radiation. It refers to the portion of the spectrum just beyond the limit of the red portion of visible radiation. The wavelengths of infrared radiation are shorter than those of radio waves and longer than those of visible light waves. They range between approximately 10 exp -6 and 10 exp -3. Infrared radiation may be detected as heat, and instruments such as bolometers are used to detect it.

We use the the infrared radiation to obtain the image of distant objects. The visible light is scattered by the atmosphere, thus it cannot help us this way. We also use infrared radiation to scout pollutants and other similar particles. In Astronomy infrared radiation is useful as well, but it cannot be that efficient because it is absorbed by water vapour, carbon dioxide and ozone.

Infrared photography is important in agriculture, industry and especially in medicine where it is used as a diagnostic tool (it can reveal pathological tissue growths). Infrared radiation plays an important role in heat transfer and is integral to the greenhouse effect,influencing the thermal radiation budget of the Earth on a global scale and affecting nearly all biospheric activity.

Visible radiation

Visible light is the most familiar form of electromagnetic radiation and makes up that portion of the spectrum to which the eye is sensitive. Visible light is only a small part of the electromagnetic spectrum. The frequencies of violet light are only about twice those of red. The corresponding wavelengths extend from 7 * 10 exp -5 centimetre (red) to 4 * 10exp -5 centimetre (violet).

Our life would not be the same or it would never exist without visible light. It represents the peak of the Sun's spectrum and close to one-half of all of its radiant energy. Visible light is essential for photosynthesis, which enables plants to produce the carbohydrates and proteins that are the food sources for animals. Coal and oil are sources of energy accumulated from sunlight in plants and microorganisms millions of years ago, and hydroelectric power is extracted from one step of the hydrologic cycle kept in motion by sunlight at the present time.

Ultraviolet radiation

This spectral region extending between visible light and X rays is designated ultraviolet. Ultraviolet radiation could be easily characterized harmful. It can cause serious damage to the tissues of our skin. The absorption of it makes our skin tan, but as the years pass by we all get to know well the danger. The main source of ultraviolet radiation is the Sun. Other sources are hot objects like mercury or xenon arc lamps. The ozone layer ban absorb almost all of this radiation. This detail makes life on Earth possible. Without it almost all life would be different, because ultraviolet radiation affects certain minerals, dyes, vitamins, natural oils, and other products. It causes them to fluoresce (they appear to glow). On the other hand this radiation is needed to our skin in order to produce vitamin D, which is indispencable for our good health.


X rays are characterized by their very short wavelengths of (10 exp -8 to 10 exp -11 centimetre), which correspond to photon energies from 200 to 100,000 eV. X rays can be absorbed by bones and tissues. Exposure of body cells and tissue to large doses of such ionizing radiation can result in abnormalities in DNA that may lead to cancer and birth defects. Although extremely harmful to our skin is considered necessary in medicine and especially in orthopedic and dental medicine.

X rays were discovered accidentally in 1895 by the German physicist Wilhelm Conrad Roentgen when he noticed during one of his experiments a strange fluorescent light. After conducting further experiments, he determined that the fluorescence was caused by visible radiation of a more penetrating nature than ultraviolet rays. He named the invisible radiation "X ray" because of its unknown nature. Subsequently, X rays were known also as Roentgen rays in his honor.

X-ray astronomy has revealed very strong sources of X rays in deep space. In the Milky Way Galaxy, of which the solar system is a part, the most intense sources are certain double star systems in which one of the two stars is thought to be either a compact neutron star or a black hole. The ionized gas of the circling companion star falls by gravitation into the compact star, generating X rays that may be more than 1,000 times as intense as the total amount of light emitted by the Sun. At the moment of their explosion, supernovae emit a good fraction of their energy in a burst of X rays.

Gamma rays

Gamma rays are the most penentrating radiation.Gamma-ray photons are between 10,000 and 10,000,000 times more energetic than the photons of visible light when they originate from radioactive atomic nuclei. Gamma rays with a million million times higher energy make up a very small part of the cosmic rays that reach the Earth from supernovae or from other galaxies. The origin of the most energetic gamma rays is not yet known.

The great penetrating power of gamma rays stems from the fact that they have no electric charge and thus do not interact with matteras strongly as do charged particles. Because of their penetrating power gamma rays can be used for radiographing holes and defects in metal castings and other structural parts. At the same time, this property makes gamma rays extremely hazardous. The lethal effect of this form of ionizing radiation makes it useful for sterilizing medical supplies that cannot be sanitized by boiling or for killing organisms that cause food spoilage. More than 50 percent of the ionizing radiation to which humans are exposed comes from natural radon gas, which is an end product of the radioactive decay chain of natural radioactive substances in minerals. Radon escapes from the ground and enters the environment in varying amounts.