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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.
In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment with an evacuated tube subjected to a high voltage.
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 1900 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 1910, British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914, Ernest Rutherford (who had named them gamma rays in 1903 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.
========,2,Range of the spectrum.
Electromagnetic waves are typically described by any of the following three physical properties: the frequency "f", wavelength λ, or photon energy "E".
Frequencies observed in astronomy range from (1 GeV gamma rays) down to the local plasma frequency of the ionized interstellar medium (~1 kHz).
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:
***LIST***.
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.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm.
A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm.
Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device.
Spectroscopes are widely used in astrophysics.
For example, many hydrogen atoms emit a radio wave photon that has a wavelength of 21.12 cm.
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 have been detected from astrophysical sources.
========,2,Rationale for spectrum regional names.
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.
========,2,Types of radiation.
========,3,Boundaries.
A discussion of the regions (or bands or types) of the electromagnetic spectrum is given below.
Note that 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.
========,3,Regions of the spectrum.
The types of electromagnetic radiation are broadly classified into the following classes:
***LIST***.
This classification goes in the increasing order of wavelength, which is characteristic of the type of radiation.
While, in general, the classification scheme is accurate, in reality there is often some overlap between neighboring types of electromagnetic energy.
For example, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power, although the latter is, in the strict sense, not electromagnetic radiation at all (see near and far field).
The distinction between X-rays and gamma rays is partly based on sources: the photons generated from nuclear decay or other nuclear and subnuclear/particle process, are always termed gamma rays, whereas X-rays are generated by electronic transitions involving highly energetic inner atomic electrons.
In general, nuclear transitions are much more energetic than electronic transitions, so gamma-rays are more energetic than X-rays, but exceptions exist.
By analogy to electronic transitions, muonic atom transitions are also said to produce X-rays, even though their energy may exceed , whereas there are many (77 known to be less than ) low-energy nuclear transitions (e.g., the nuclear transition of thorium-229), and, despite being one million-fold less energetic than some muonic X-rays, the emitted photons are still called gamma rays due to their nuclear origin.
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.
Quite often, in high energy physics and in medical radiotherapy, very high energy EMR (in the >10 MeV region)—which is of higher energy than any nuclear gamma ray—is not called X-ray or gamma-ray, but instead by the generic term of "high energy photons."
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.
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.
========,3,Radio frequency.
Radio waves generally are utilized by antennas of appropriate size (according to the principle of resonance), with wavelengths ranging from hundreds of meters to about one millimeter.
They are used for transmission of data, via modulation.
Television, mobile phones, wireless networking, and amateur radio all use radio waves.
The use of the radio spectrum is regulated by many governments through frequency allocation.
Radio waves can be made to carry information by varying a combination of the amplitude, frequency, and phase of the wave within a frequency band.
When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material.
This effect (the skin effect) is used in antennas.
========,3,Microwaves.
The super-high frequency (SHF) and extremely high frequency (EHF) of microwaves are on the short side of radio waves.
Microwaves are waves that are typically short enough (measured in millimeters) to employ tubular metal waveguides of reasonable diameter.
Microwave energy is produced with klystron and magnetron tubes, and with solid state diodes such as Gunn and IMPATT devices.
Microwaves are absorbed by molecules that have a dipole moment in liquids.
In a microwave oven, this effect is used to heat food.
Low-intensity microwave radiation is used in Wi-Fi, although this is at intensity levels unable to cause thermal heating.
Volumetric heating, as used by microwave ovens, transfers energy through the material electromagnetically, not as a thermal heat flux.
The benefit of this is a more uniform heating and reduced heating time; microwaves can heat material in less than 1% of the time of conventional heating methods.
When active, the average microwave oven is powerful enough to cause interference at close range with poorly shielded electromagnetic fields such as those found in mobile medical devices and poorly made consumer electronics.
========,3,Terahertz radiation.
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.
========,3,Infrared radiation.
The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz to 400 THz (1 mm - 750 nm).
It can be divided into three parts:
***LIST***.
========,3,Visible radiation (light).
Above infrared in frequency comes visible light.
The Sun emits its peak power in the visible region, although integrating the entire emission power spectrum through all wavelengths shows that the Sun emits slightly more infrared than visible light.
By definition, visible light is the part of the EM spectrum the human eye is the most sensitive to.
Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another.
This action allows the chemical mechanisms that underlie human vision and plant photosynthesis.
The light that excites the human visual system is a very small portion of the electromagnetic spectrum.
A rainbow shows the optical (visible) part of the electromagnetic spectrum; infrared (if it could be seen) would be located just beyond the red side of the rainbow with ultraviolet appearing just beyond the violet end.
Electromagnetic radiation with a wavelength between 380 nm and 760 nm (400–790 terahertz) is detected by the human eye and perceived as visible light.
Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant.
White light is a combination of lights of different wavelengths in the visible spectrum.
Passing white light through a prism splits it up into the several colors of light observed in the visible spectrum between 400 nm and 780 nm.
If radiation having a frequency in the visible region of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes the eyes, this results in visual perception of the scene.
The brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this insufficiently-understood psychophysical phenomenon, most people perceive a bowl of fruit.
At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses.
Natural sources produce EM radiation across the spectrum, and technology can also manipulate a broad range of wavelengths.
Optical fiber transmits light that, although not necessarily in the visible part of the spectrum (it is usually infrared), can carry information.
The modulation is similar to that used with radio waves.
========,3,Ultraviolet radiation.
Next in frequency comes ultraviolet (UV).