The Character of the General or Continuous Spectrum Radiation
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The electromagnetic spectrum is the range of frequencies the spectrum of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from below one hertz to above 10 25 hertz, corresponding to wavelengths from thousands of kilometers down to a fraction of the size of an atomic nucleus.
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This frequency range is divided into separate bands, and the electromagnetic waves within each frequency band are called by different names; beginning at the low frequency long wavelength end of the spectrum these are: radio waves , microwaves , infrared , visible light , ultraviolet , X-rays , and gamma rays at the high-frequency short wavelength end.
The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. The limit for long wavelengths is the size of the universe itself, while it is thought that the short wavelength limit is in the vicinity of the Planck length. In most of the frequency bands above, a technique called spectroscopy can be used to physically separate waves of different frequencies, producing a spectrum showing the constituent frequencies.
Spectroscopy is used to study the interactions of electromagnetic waves with matter. For most of history, visible light was the only known part of the electromagnetic spectrum. The ancient Greeks recognized that light traveled in straight lines and studied some of its properties, including reflection and refraction. The study of light continued, and during the 16th and 17th centuries conflicting theories regarded light as either a wave or a particle.
Transmission, Reflection, and Absorption
The first discovery of electromagnetic radiation other than visible light came in , when William Herschel discovered infrared radiation. He noticed that the highest temperature was beyond red. He theorized that this temperature change was due to "calorific rays", a type of light ray that could not be seen. The next year, Johann Ritter , working at the other end of the spectrum, noticed what he called "chemical rays" invisible light rays that induced certain chemical reactions. These behaved similarly to visible violet light rays, but were beyond them in the spectrum.
Electromagnetic radiation was first linked to electromagnetism in , when Michael Faraday noticed that the polarization of light traveling through a transparent material responded to a magnetic field see Faraday effect. During the s James Maxwell developed four partial differential equations for the electromagnetic field. Two of these equations predicted the possibility and behavior of waves in the field. Analyzing the speed of these theoretical waves, Maxwell realized that they must travel at a speed that was about the known speed of light.
This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave. Maxwell's equations predicted an infinite number of frequencies of electromagnetic waves , all traveling at the speed of light. This was the first indication of the existence of the entire electromagnetic spectrum. Maxwell's predicted waves included waves at very low frequencies compared to infrared, which in theory might be created by oscillating charges in an ordinary electrical circuit of a certain type.
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Attempting to prove Maxwell's equations and detect such low frequency electromagnetic radiation, in the physicist Heinrich Hertz built an apparatus to generate and detect what are now called radio waves. Hertz found the waves and was able to infer by measuring their wavelength and multiplying it by their frequency that they traveled at the speed of light. Hertz also demonstrated that the new radiation could be both reflected and refracted by various dielectric media, in the same manner as light.
For example, Hertz was able to focus the waves using a lens made of tree resin. In a later experiment, Hertz similarly produced and measured the properties of microwaves. These new types of waves paved the way for inventions such as the wireless telegraph and the radio. He called these radiations x-rays and found that they were able to travel through parts of the human body but were reflected or stopped by denser matter such as bones.
Before long, many uses were found for them in the field of medicine. The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In Paul Villard was studying the radioactive emissions of radium when he identified a new type of radiation that he first thought consisted of particles similar to known alpha and beta particles, but with the power of being far more penetrating than either. However, in , British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in , Ernest Rutherford who had named them gamma rays in when he realized that they were fundamentally different from charged alpha and beta particles and Edward Andrade measured their wavelengths, and found that gamma rays were similar to X-rays, but with shorter wavelengths and higher frequencies.
Frequencies observed in astronomy range from 2. Wavelength is inversely proportional to the wave frequency,  so gamma rays have very short wavelengths that are fractions of the size of atoms , whereas wavelengths on the opposite end of the spectrum can be as long as the universe. Photon energy is directly proportional to the wave frequency, so gamma ray photons have the highest energy around a billion electron volts , while radio wave photons have very low energy around a femtoelectronvolt.
These relations are illustrated by the following equations:. Whenever electromagnetic waves exist in a medium with matter , their wavelength is decreased.
Wavelengths of electromagnetic radiation, no matter what medium they are traveling through, are usually quoted in terms of the vacuum wavelength , although this is not always explicitly stated. Generally, electromagnetic radiation is classified by wavelength into radio wave , microwave , terahertz or sub-millimeter radiation, infrared , the visible region that is perceived as light, ultraviolet , X-rays and gamma rays. The behavior of EM radiation depends on its wavelength. When EM radiation interacts with single atoms and molecules, its behavior also depends on the amount of energy per quantum photon it carries.
Introduction to electromagnetic waves
Spectroscopes are widely used in astrophysics. For example, many hydrogen atoms emit a radio wave photon that has a wavelength of Also, frequencies of 30 Hz and below can be produced by and are important in the study of certain stellar nebulae  and frequencies as high as 2. The types of electromagnetic radiation are broadly classified into the following classes regions, bands or types : .
This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation. There are no precisely defined boundaries between the bands of the electromagnetic spectrum; rather they fade into each other like the bands in a rainbow which is the sub-spectrum of visible light. Radiation of each frequency and wavelength or in each band has a mix of properties of the two regions of the spectrum that bound it. For example, red light resembles infrared radiation in that it can excite and add energy to some chemical bonds and indeed must do so to power the chemical mechanisms responsible for photosynthesis and the working of the visual system.
By analogy to electronic transitions, muonic atom transitions are also said to produce X-rays, even though their energy may exceed 6 megaelectronvolts 0. The convention that EM radiation that is known to come from the nucleus, is always called "gamma ray" radiation is the only convention that is universally respected, however. Many astronomical gamma ray sources such as gamma ray bursts are known to be too energetic in both intensity and wavelength to be of nuclear origin. The region of the spectrum where a particular observed electromagnetic radiation falls, is reference frame -dependent due to the Doppler shift for light , so EM radiation that one observer would say is in one region of the spectrum could appear to an observer moving at a substantial fraction of the speed of light with respect to the first to be in another part of the spectrum.
For example, consider the cosmic microwave background. It was produced when matter and radiation decoupled, by the de-excitation of hydrogen atoms to the ground state.
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These photons were from Lyman series transitions, putting them in the ultraviolet UV part of the electromagnetic spectrum. Now this radiation has undergone enough cosmological red shift to put it into the microwave region of the spectrum for observers moving slowly compared to the speed of light with respect to the cosmos.
Electromagnetic radiation interacts with matter in different ways across the spectrum. These types of interaction are so different that historically different names have been applied to different parts of the spectrum, as though these were different types of radiation. Thus, although these "different kinds" of electromagnetic radiation form a quantitatively continuous spectrum of frequencies and wavelengths, the spectrum remains divided for practical reasons related to these qualitative interaction differences. Radio waves are emitted and received by antennas , which consist of conductors such as metal rod resonators.
In artificial generation of radio waves, an electronic device called a transmitter generates an AC electric current which is applied to an antenna. The oscillating electrons in the antenna generate oscillating electric and magnetic fields that radiate away from the antenna as radio waves.
In reception of radio waves, the oscillating electric and magnetic fields of a radio wave couple to the electrons in an antenna, pushing them back and forth, creating oscillating currents which are applied to a radio receiver. Earth's atmosphere is mainly transparent to radio waves, except for layers of charged particles in the ionosphere which can reflect certain frequencies.
Radio waves are extremely widely used to transmit information across distances in radio communication systems such as radio broadcasting , television , two way radios , mobile phones , communication satellites , and wireless networking. In a radio communication system, a radio frequency current is modulated with an information-bearing signal in a transmitter by varying either the amplitude, frequency or phase, and applied to an antenna. The radio waves carry the information across space to a receiver, where they are received by an antenna and the information extracted by demodulation in the receiver.follow
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Radio waves are also used for navigation in systems like Global Positioning System GPS and navigational beacons , and locating distant objects in radiolocation and radar. They are also used for remote control , and for industrial heating. The use of the radio spectrum is strictly regulated by governments, coordinated by a body called the International Telecommunications Union ITU which allocates frequencies to different users for different uses.
Microwaves are radio waves of short wavelength , from about 10 centimeters to one millimeter, in the SHF and EHF frequency bands. Although they are emitted and absorbed by short antennas, they are also absorbed by polar molecules , coupling to vibrational and rotational modes, resulting in bulk heating. Unlike higher frequency waves such as infrared and light which are absorbed mainly at surfaces, microwaves can penetrate into materials and deposit their energy below the surface. This effect is used to heat food in microwave ovens , and for industrial heating and medical diathermy.
5.2: Light, Particles, and Waves
Microwaves are the main wavelengths used in radar , and are used for satellite communication , and wireless networking technologies such as Wi-Fi , although this is at intensity levels unable to cause thermal heating. The copper cables transmission lines which are used to carry lower frequency radio waves to antennas have excessive power losses at microwave frequencies, and metal pipes called waveguides are used to carry them.
Although at the low end of the band the atmosphere is mainly transparent, at the upper end of the band absorption of microwaves by atmospheric gasses limits practical propagation distances to a few kilometers. Terahertz radiation is a region of the spectrum between far infrared and microwaves. Until recently, the range was rarely studied and few sources existed for microwave energy at the high end of the band sub-millimeter waves or so-called terahertz waves , but applications such as imaging and communications are now appearing.
Scientists are also looking to apply terahertz technology in the armed forces, where high-frequency waves might be directed at enemy troops to incapacitate their electronic equipment. It can be divided into three parts: . Above infrared in frequency comes visible light.