What is Electromagnetic Spectrum:
The word spectrum comes from a Latin word meaning “form” or “appearance”. Other familiar words from the same root include “spectacle” and “species.” Newton introduced word to describe the rainbow-like image that resulted when a beam of sunlight passed through a glass prism. Today we speak of the electromagnetic spectrum to indicate the many different kinds of electromagnetic radiation,classified according to their frequency or wavelength on a scale from large to small. We also speak of the political views on a scale from ultraconservative to ultra-liberal.
In Maxwell’s time, light and the adjoining infrared and ultraviolet radiations were the only known types of electromagnetic radiations. Today the electromagnetic spectrum,includes a board range of different kinds of radiations from a variety of sources. From Maxwell’s theory we conclude that, even through these radiations differ greatly in their properties, in their means of production,and in this ways we observe them, they share other features in common: they call can be described in terms of electric and magnetic fields,and they all travel through vacuum with the same speed (the speed of light). In fact, from the fundamental point of view. they differ only in wavelength or frequency. The names given to the various region of the spectrum. I have to do only with the way the different types of waves are produced or observed; they have nothing to do with any fundamental property of the waves. Other than the difference in their wavelengths, there is no experimental way to distinguish a wave in the visible region from one in the infrared region; the waves have identical forms and identical mathematical descriptions. There are no gaps in the spectrum, nor are the sharp boundaries between the various categories. (Certain regions of the spectrum are assigned by law for commercial or other uses, such as TV . AM . or FM broadcasting).
Let us consider some of these types of electromagnetic radiations in more detail.
- Light. The visible region of the spectrum is the one most familiar to us, because as a species we have adapted receptors (eyes) that are sensitive to the most intense electromagnetic radiation emitted by the Sun, the closest extraterrestrial source. The limits of the wavelength of the visible region are from about 400 nm (violet) to about 700 nm (red). Light is often emitted when the outer (or valence) electrons in atoms change their state of motion: for this reason. Such transitions in the state of electron are called optical transitions. the color of the light tells us some thing about the atoms or the object from which it was emitted. The study of the light emitted from the Sun and from distant stars gives information about their composition.
- Infrared. infrared radiation, which has wavelengths longer than the visible (from 0.7 μm to about 1 mm), is commonly emitted by atoms or molecules when they change their rotational or vibrational motion. Often this change occurs as a change in the internal energy of the emitting object and is observed as a change in the internal energy of the object that detects the radiation. In this case, infrared radiation is an important means of heat transfer and is sometimes called heat radiation. The warmth you feel when you place you hand near a glowing light bulb is primarily a result of the infrared radiation emitted from the bulb and absorbed by your hand. All objects emit electromagnetic radiation ( called ” thermal radiation;” of the extended text) because of their temperature. Objects of temperatures in the range we normally encounter (say. 3 K to 300 K )emit their most intense thermal radiation in the infrared region of the spectrum. Mapping the infrared radiation from space has give us information that supplements that obtained from the visible radiation.
- Microwaves. Microwaves can be regarded as short radio waves,with typical wavelengths in the range 1 mm to 1 m. They are commonly produced by electromagnetic oscillators in electric circuits. As in this case of microwave ovens. Microwaves are often used to transmit telephone conservation: show a micro wave station that serves to relay telephone calls. Microwaves also reach us from extraterrestrial sources. The most abundant component is the microwave background radiation, which is believed to be the electromagnetic radiation associated with the “Big Bang” fireball that marked the birth of the universe some 1010 years ago: as the universe expanded and cooled, the wavelength of this radiation was stretched until it is now in the microwave region, with a peak wave, length of about 1 mm. Neutral hydrogen atoms, which populate the region between the stars in our galaxy, are another common extraterrestrial source of microwaves, emitting radiation with a wavelength of 21 cm.
- Radio Waves. Radio waves have wavelengths longer than 1 m. They are produced from terrestrial sources through electrons oscillating in wires of electric circuits. By carefully choosing the geometry of these circuits, as in an antenna. We can control the distribution in space of the emitted radiation (if the antenna acts as a transmitter) or the sensitive of the detector (if the antenna acts as a receiver). Traveling outward at the speed of light, the expanding wave-front of TV signals transmitted on Earth since about 1950 has now reached approximately 400 stars, carrying information to their inhabitants, if any, about our civilization.
Radio waves reach us from extraterrestrial sources, the Sun being a major source that often interferes with radio o TV reception on Earth. Jupiter is also an active source of radio emission. Mapping the radio emission from extraterrestrial sources. Known as radio astronomy, has provided information about the universe that is often not obtainable information about the universe that is often not obtainable using optical telescopes. Furthermore, because the Earth’s atmosphere does not absorb strongly at radio wavelengths, radio astronomy provides certain advantages over optical, infrared, or microwave astronomy on Earth. A typical result of the observation of our galaxy at radio wavelengths. One of the most startling discoveries of radio astronomy was the existence of pulsed sources of radio waves, first observed in 1968. These objects, known as pulsars,emit very short bursts of radio waves separated in time by intervals of the order of seconds.This time interval between pulses is extremely stable,varying by less than 10-9s. Pulsars are believed tp originate from rotating neutron stars,in which electrons trapped by the magnetic field experience large centripetal accelerations owing to the rotation.The highly directional radio emissions sweep by the earth like a searchlight beacon as the star rotates.Pulsars have been observed over the full range of the spectrum,including visible and x ray wavelengths.
The radiations of wavelengths shorter than the visible begin with the ultraviolet (1 nm to 400 nm),which can be produced in atomic transitions of the outer electrons as well as in radiation from thermal sources such as the sun.Because outer atmosphere absorbs strongly at ultraviolet wavelengths,little of this radiation from the sun to the ground.However,the principal agent of this absorption is atmospheric ozone,which has been depleted in recent years as a result of chemical reactions with fluorocarbons released aerosol sprays,refrigeration equipment ,and other sources.Brief exposure to ultraviolet radiation causes common sunburn,but long term exposure can lead to more serious effects,including skin cancer.Ultraviolet astronomy is done using observatories carried into earth orbit by satellites.
X -rays (typical wavelength 0.01 nm to 10 nm) can be produced with discrete wavelengths in individual transitions among the inner (most tightly bound) electrons of an atom,and they can also be produced when charged particles (such as electrons ) are decelerated.X -ray wavelengths correspond roughly to the spacing between the atoms of solids;therefore scattering of x rays from materials is a useful way of studying their structure.X rays can easily penetrate soft issue but are stopped by bone and other solid matter,for this reason they have found wide use in medical diagnosis. X ray astronomy,like ultraviolet astronomy,is done with orbiting observatories.Most stars,such as the sun,are not strong x ray emitters;however,in certain systems consisting of two nearby stars orbiting about their common center of mass (called a binary system),material from one star can be heated and accelerated as it falls into the other,emitting x rays in the process.Although confirming evidence is not yet available,it is believed that the more massive member of certain x ray binaries may be a black hole.
Gama rays are electromagnetic radiations with the shortest wavelengths (less than 10 pm).They are the most penetrating of electromagnetic radiations,and exposure to intense gamma radiation can have a harmful effect on the human body.These radiations can be emitted in transitions of an atomic nucleus from one state to another and can also occur in the decays of certain elementary particles;for example,a neutral pion can decay into two gamma rays according to π° → γ + ϒ and an electron and a positron ( the antiparticle of the electron) can mutually annihilate into two gamma rays: e – + e + → γ + γ In general,each such process emits gamma rays of a unique wavelength.In gamma ray astronomy detection of such radiations (and measurement of their wavelength) serves as evidence of particular nuclear processes in the universe. Watch also: