#### The electromagnetic spectrum, Chemistry tutorial

Introduction

The electromagnetic spectrum is the range of all feasible frequencies of electromagnetic radiation. The 'electromagnetic spectrum' of an object is the traits distribution of electromagnetic radiation emitted or absorbed via that object.

The electromagnetic spectrum expands from low particular frequencies utilized for modern radio contact to gamma radiation at the short-wavelength (high-frequency) end, thereby covering wavelengths from thousands of kilometres downward to a fraction of the size of an atom. It is for this reason that the electromagnetic spectrum is extremely studied for spectroscopic reasons to distinguish matter. The limit for long wavelength is the size of the universe itself, while it is deliberation that the short wavelength limit is in the vicinity of the Planck length, even though in principle the spectrum is infinite and continuous.

Range of the spectrum

Electromagnetic waves are classically explained via any of the subsequent 3 physical properties: the frequency f, wavelength λ, or photon energy E. Frequencies range from 2.4×1023 Hz (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 can be as long as the universe. Photon  energy is straight proportional to the wave frequency, so gamma rays contain the highest energy (around a billion electron volts) and radio waves have extremely low energy (around a femto-electronvolt). Such relations are demonstrated via the subsequent equations:

F=c/ λ, or f= E/h, or                   E= hc/ λ

Where:

• c = 299,792,458 m s-1 is the speed of light in vacuum
• h = 6.62606896(33) ×10-34

J s = 4.13566733(10) ×10-15 eV s is Planck's constant.

Whenever electromagnetic waves exist in a medium through matter, their wavelength is reduced. Wavelengths of electromagnetic radiation, no matter what medium they are travelling through, are generally quoted in terms of the vacuum wavelength, even though this isn't always explicitly stated. Usually electromagnetic radiation is classified via wavelength into radio wave, microwave, terahertz (or sub-millimeter) radiation, infrared, the visible region we recognize as light, ultraviolet, X-rays and gamma rays. The behaviour of EM radiation based on its wavelength. Whenever EM radiation contacts by single atoms and molecules, its behaviour as well based on the amount of energy per quantum (photon) it carries.

Spectroscopy can notice a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. An ordinary laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of things, gases, or even stars can be attained from this kind of device. Spectroscopes are extensively utilized in astrophysics. For instance, many hydrogen atoms emit a radio wave photon that has a wavelength of 21.12 cm. As well, frequencies of 30 Hz and below can be created via and are significant in the study of assured stellar nebulae and frequencies as high as 2.9×1027 Hz have been detected from astrophysical sources.

Interaction of electromagnetic Radiation with Matter

Electromagnetic radiation interacts by matter in different ways in different parts of the spectrum. The kinds of interaction can be so different that it seems to be justified to pass on to different types of radiation. At the same time, there is a continuum enclosing all such 'different kinds' of electromagnetic radiation. Therefore we refer to a spectrum, but split it up depends on the different interactions through matter.

Fig: Interaction of electromagnetic Radiation with Matter

Figure: The electromagnetic spectrum

The types of electromagnetic radiation are usually classified into the following classes:

This categorization goes in the rising order of wavelength, which is trait of the kind of radiation. While, in general, the classification scheme is precise, in reality there is frequently several overlap between neighboring kinds of electromagnetic energy. For instance, SLF radio waves at 60 Hz may be received and studied by astronomers, or may be ducted along wires as electric power, even though 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 depend on sources: gamma rays are the photons generated from  nuclear decay or other nuclear and sub nuclear/particle process, whereas X-rays are produced via electronic transitions including extremely energetic inner atomic electrons. In common, nuclear transitions are much more vigorous than electronic changes, so gamma-rays are more energetic than X-rays, but exceptions exist. By analogy to electronic transitions, muonic atom transitions are too called to produce X-rays, even though their energy might exceed 6 megaelectronvolts (0.96 pJ), whereas there are many (77 recognized to be less than 10 keV (1.6 fJ)) low-energy nuclear transitions (for example, the 7.6 eV (1.22 aJ) nuclear transition of thorium-229), and, despite being one million-fold less energetic than several muonic X-rays, the released photons are still said gamma rays due to their nuclear origin.

As well, the region of the spectrum of the exacting electromagnetic radiation is situation frame-dependent (on account of the Doppler shift for light), so EM radiation that one observer would say is in one region of the spectrum could show to an observer moving at a substantial fraction of the speed of light through respect to the 1st to be in another part of the spectrum. For instance, consider the cosmic microwave background. It was created, when matter and radiation decoupled, via the de-excitation of hydrogen atoms to the ground state. Such photons were from Lyman series transitions, putting them in the ultraviolet (UV) part of the electromagnetic spectrum. Now this radiation has undergone sufficient 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. Though, for elements moving near the speed of light, this radiation will be blue-shifted in their rest frame. The highest-energy cosmic ray protons are moving such that, in their rest frame, this radiation is blueshifted to high-energy gamma rays, which interact through the proton to create bound quark-antiquark pairs (pions).

Radio waves usually are utilized via antennas of appropriate size (according to the standard of resonance), through wavelengths ranging from hundreds of meters to about one millimeter. They are utilized 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 fluctuating an amalgamation of the amplitude, frequency, and phase of the wave inside 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 via exciting the electrons of the conducting stuff. This consequence (the skin result) is utilized in antennas.

Microwaves

The super-high frequency (SHF) and extremely elevated frequency (EHF) of microwaves come after radio waves. Microwaves are waves that are classically short sufficient to employ tubular metal waveguides of reasonable diameter. Microwave energy is created through klystron and magnetron tubes and by solid state diodes these as Gunn and IMPATT devices.

Microwaves are absorbed via molecules, which have a dipole moment in liquids. In a microwave oven, this consequence is utilized to heat food. Low-intensity microwave radiation is employed in Wi-Fi, even though this is at intensity levels unable to reason thermal heating. Volumetric heating, as utilized through microwave ovens, conveys energy through the substance electromagnetically, not as a thermal heat flux. The advantage of this is a more uniform heating and decreased heating time; microwaves can heat substance in less than 1% of the time of conventional heating techniques. When active, the average microwave oven is powerful adequate to cause interference at close range through inadequately protected electromagnetic fields such as those originate in mobile medical machines and cheap consumer electronics.

The infrared part of the electromagnetic spectrum wraps the range from approximately 300 GHz (1 mm) to 400 THz (750 nm). It can be separated into 3 parts:

1.  Far-infrared, from 300 GHz (1 mm) to 30 THz (10 m). The lower part of this range might as well be said microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, via molecular motions in liquids, and through phonons in solids. The water in Earth's atmosphere absorbs so powerfully in this range that it renders the atmosphere in effect opaque. However, there are certain wavelength ranges ("windows") inside the opaque range that permit partial transmission, and can be utilized for astronomy. The wavelength range from approximately 200 m up to a few mm is frequently termed to as 'sub-millimeter' in astronomy, reserving far infrared for wavelengths below 200 m.

2.  Mid-infrared, from 30 to 120 THz (10 to 2.5 m). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed via molecular vibrations, where the dissimilar atoms in a molecule vibrate around their equilibrium situations. This range is sometimes said the fingerprint region because the mid-infrared absorption spectrum of a complex is extremely exact for that compound.

3.  Near-infrared, from 120 to 400 THz (2,500 to 750 nm). Physical procedures that are appropriate for this range are similar to those for visible light.

Above infrared in frequency comes visible light. This is the range in that the sun and other stars release most of their emission and the spectrum that the human eye is the most sensitive to. Visible light (and near-infrared light) is classically absorbed and released via electrons in molecules and atoms that move from one energy level to another. The light we see by our eyes is actually an extremely small portion of the electromagnetic spectrum. A rainbow illustrates the optical (visible) part of the electromagnetic spectrum; infrared (if we could see it) would be traced just beyond the red side of the rainbow through ultraviolet appearing just ahead of the violet end.

Electromagnetic radiation by a wavelength between 380 nm and 760 nm (790-400 terahertz) is noticed via the human eye and perceived as visible light. Other wavelengths, particularly near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are as well sometimes termed to as light, particularly when the visibility to humans isn't relevant. White light is an amalgamation of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up in to the several colours of light observed in the visible spectrum between 400 nm and 780 nm.

If radiation having a frequency in the observable area of the EM spectrum reflects off an object, say, a bowl of fruit, and then strikes our eyes, this consequences in our illustration perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and throughout this insufficiently-understood psychophysical phenomenon, most people perceive a bowl of fruit.

At most wavelengths, though, the information carried via electromagnetic radiation isn't straight noticed via human senses. Natural sources produce EM radiation across the spectrum, and our technology can as well manipulate a wide range of wavelengths. Optical fibre transmits light that, even though not essentially in the visible part of the spectrum, can hold information. The modulation is similar to that utilized through radio waves.

Ultraviolet light

Subsequently in frequency approaches ultraviolet (UV). The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray. UV in the extremely shortest range (next to X-rays) is able even of ionizing atoms (see photoelectric consequence), really changing their physical behavior. At the middle range of UV, UV rays can't ionize but can split chemical bonds, making molecules to be unusually reactive. Sunburn, for instance, is caused via the disruptive results of middle range UV radiation on skin cells that is the major cause of skin cancer. UV rays in the middle range can irreparably injure the complex DNA molecules in the cells producing thymine dimers making it an extremely potent mutagen.

The sun releases a huge amount of UV radiation that could potentially turn Earth's land surface into a barren desert (even though ocean water would give several protections for life there). Though, most of the Sun's most-damaging UV wavelengths are absorbed through the atmosphere's nitrogen, oxygen, and ozone layer before they reach the surface. The higher ranges of UV (vacuum UV) are absorbed via nitrogen and, at longer wavelengths, through easy diatomic oxygen in the air. Most of the UV in this mid-range is blocked via the ozone layer that absorbs powerfully in the significant 200-315 nm range, the lower part of which is also long to be absorbed via ordinary oxygen in air. The range between 315 nm and visible light (said UV-A) isn't blocked well via the atmosphere, but doesn't cause sunburn and does less biological damage. Though, it isn't harmless and does cause oxygen radicals, mutation and skin harm. See ultraviolet for more information.

X-rays

After UV come X-rays, which, like the upper ranges of UV are as well ionizing. Yet, due to their higher energies, X-rays can as well interact through matter via means of the Compton Effect. Hard X-rays have shorter wavelengths than soft X-rays. As they can pass through most substances, X-rays can be utilized to 'see through' objects, the most notable use being diagnostic X-ray images in medicine (a process known as radiography) in addition to for high-energy physics and astronomy. Neutron stars and accretion disks around black holes emit X-rays, which enable us to study them. X-rays are given off via stars and are strongly emitted via several kinds of nebulae.

Gamma rays

After hard X-rays come gamma rays that were discovered via Paul Villard in the year 1900. Such are the most energetic photons, having no described lower limit to their wavelength. They are useful to astronomers in learn of high-energy objects or regions, and discover an employ by physicists thanks to their penetrative ability and their production from radioisotopes. Gamma rays are as well utilized for the irradiation of food and seed for sterilization, and in medicine they are utilized in radiation cancer therapy and various types of diagnostic imaging these as PET scan. The wavelength of gamma rays can be computed by elevated accuracy via means of Compton scattering.

As we know that there are no accurately described boundaries between the bands of the electromagnetic spectrum. Radiations of several kinds contain a mixture of the properties of those in two regions of the spectrum. For instance red light resembles infrared radiation in, which it can resonate several chemical bonds.

Nature of Light and Quantum Theory

The early theories describing the atomic structure are depends on conventional physics. Nevertheless such theories couldn't describe the behaviour of atom completely. The modern view of atomic structure is based on quantum theory introduced via Max Planck. Light is considered as an electromagnetic radiation. It consists of 2 components for example, the electric component and the magnetic component that oscillate perpendicular to each other in addition to the direction of path of radiation.

The electromagnetic radiations are produced by the vibrations of a charged particle. The properties of light can be explained by considering it as either wave or particle as follows.

Wave nature of light

According to the wave theory proposed by Christiaan Huygens, light is considered to be emitted as a series of waves in all directions. The following properties can be defined for light by considering the wave nature.

Wavelength (λ): The distance between 2 successive similar points on a wave is said as wavelength. It is symbolized by λ. Units: cm, Angstroms (Ao), nano meters (nm), milli microns (mµ) and so on,

Note:  1 Ao = 10-8 cm.

1 nm= 10-9m = 10-7cm

Frequency (ν): The number of vibrations done by a particle in unit time is called frequency. It is denoted by 'ν'.

Units: cycles per second = Hertz = sec-1

Velocity (c): Velocity is described as the distance covered via the wave in unit time. It is denoted by 'c'.

V=D/T

Velocity of light = c = 3.0 x 108 m.sec-1

= 3.0 x 1010 cm.sec-1

As we know: For all kinds of electromagnetic radiations, the velocity is a steady value. The relation between velocity (c), wavelength (λ) and frequency (ν) can be specified via subsequent equation:  velocity = frequency x wavelength (c = νλ).

Wave number (v˜): The number of waves spread in a length of one centimeter is said wave number. It is symbolized by. It is the reciprocal of wavelength, λ.

v˜=1/ λ

units: cm-1, m-1

Amplitude: The distance from the midline to the peak or the trough is termed amplitude of the wave. It is usually symbolized via 'A' (a variable). Amplitude is a compute of the intensity or brightness of light radiation.

Particle nature of light

Though most of the properties of light can be understood via considering it as a wave, several of the properties of light can only be described via using particle (corpuscular) nature of it. Newton considered light to possess particle nature. In the year 1900, in order to describe black body radiations, Max Planck proposed Quantum theory via considering light to possess particle nature.

Planck's quantum theory

Black body: The object that absorbs and releases the radiation of energy wholly is said a black body. Basically it isn't possible to construct a perfect black body. But a hollow metallic sphere coated inside through platinum black by a small aperture in its wall can perform as a near black body. When the black body is warmth to elevated temperatures, it releases radiations of dissimilar wavelengths.

The subsequent curves are attained when the intensity of radiations are plotted against the wavelengths, at dissimilar temperatures. Subsequent are the conclusions that can be drawn from above graphs.

1) At a specified temperature, the intensity of radiation increases with wavelength and reaches a maximum value and then starts decreasing.

2) With enhance in temperature, the wavelength of maximum intensity (λmax) shifts towards lower wavelengths. According to classical physics, energy should be released incessantly and the intensity should enhance through rise in temperature. The curves should be as given by dotted line.

In order to describe above experimental observations Max Planck proposed the subsequent theory.

Quantum theory:

1) Energy is emitted due to vibrations of charged particles in the black body.

2) The radiation of energy is released or absorbed discontinuously in the form of small   discrete energy packets called quanta.

3) Each quantum is connected through definite amount of energy that is specified via the equation

E = hν

Where h = Planck's constant = 6.625 x 10-34 J. sec = 6.625 x10-27 erg. sec

4) The total energy of radiation is quantized i.e., the total energy is an integral multiple of hν. It can only have the values of 1 hν or 2 hν or 3 hν. It cannot be the fractional multiple of hν.

5) Energy is released and absorbed in the form of quanta but propagated in the shape of waves.

The following table gives approximate wavelengths, frequencies, and energies for selected regions of the electromagnetic spectrum:

The notation "eV" places for electron-volts, a general unit of energy evaluate in atomic physics.

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