Physics > Electromagnetic Waves > 2.0 Electromagnetic Waves
Electromagnetic Waves
1.0 Introduction
2.0 Electromagnetic Waves
2.1 Properties of electromagnetic waves
2.2 Production of electromagnetic waves
2.3 Energy density of electromagnetic waves
2.4 Intensity of electromagnetic waves
2.5 Momentum of electromagnetic waves
2.6 Radiation pressure
2.7 Poynting vector
2.8 Electromagnetic spectrum
2.8 Electromagnetic spectrum
2.2 Production of electromagnetic waves
2.3 Energy density of electromagnetic waves
2.4 Intensity of electromagnetic waves
2.5 Momentum of electromagnetic waves
2.6 Radiation pressure
2.7 Poynting vector
2.8 Electromagnetic spectrum
The orderly distribution of electromagnetic radiations according to their wavelength or frequency is known as electromagnetic spectrum.
The electromagnetic spectrum is as shown in the figure.
We briefly describe these different types of electromagnetic waves in order of decreasing wavelengths.
2.8.1 Radio waves
Radio waves are produced by the accelerated motion of charges in conducting wires. They are used in radio and television communication systems. They are generally in the frequency range from 500 $kHz$ to about 1000 $MHz$. The AM (amplitude modulated) band is from 530 $kHz$ to 1710 $kHz$. Higher frequencies upto 54 $MHz$ are used for short wave bands. TV waves range from 54 $MHz$ to 890 $MHz$. The FM (frequency modulated) radio band extends from 88 $MHz$ to 108 $MHz$. Cellular phones use radio waves to transmit voice communication in the ultrahigh frequency (UHF) band.
2.8.2 Microwaves
Microwaves (short-wavelength radio waves), with frequencies in the gigahertz $(GHz)$ range, are produced by special vacuum tubes (called klystrons, magnetrons, and Gunn diodes). Due to their short wavelengths, they are suitable for the radar systems used in aircraft navigation. Radar also provides the basis for the speed guns used to time fast balls, tennis- serves, and automobiles. Microwave ovens are an interesting domestic application of these waves. In such ovens, the frequency of the microwaves is selected to match the resonant frequency of water molecules so that energy from the waves is transferred efficiently to the kinetic energy of the molecules. This raises the temperature of any food containing water.
2.8.3 Infrared waves
Infrared waves are produced by hot bodies and molecules. This band lies adjacent to the low-frequency or long-wave length end of the visible spectrum. Infrared waves are sometimes referred to as heat waves. This is because water molecules present in most materials readily absorb infrared waves (many other molecules, for example, $CO_2$, $NH_3$, also absorb infrared waves). After absorption, their thermal motion increases, that is, they heat up and heat their surroundings.
Infrared lamps are used in physical therapy. Infrared radiation also plays an important role in maintaining the earth’s warmth or average temperature through the greenhouse effect. Incoming visible light (which passes relatively easily through the atmosphere) is absorbed by the earth’s surface and re- radiated as infrared (longer wavelength) radiations. This radiation is trapped by greenhouse gases such as carbon dioxide and water vapour. Infrared detectors are used in Earth satellites, both for military purposes and to observe growth of crops. Electronic devices (for example semiconductor light emitting diodes) also emit infrared and are widely used in the remote switches of household electronic systems such as TV sets, video recorders and hi-fi systems.
2.8.4 Visible rays
It is the most familiar form of electromagnetic waves. It is the part of the spectrum that is detected by the human eye. It runs from about $4 \times {10^{14\;}}Hz$ to about $7 \times {10^{14\;}}Hz$ or a wavelength range of about $700 - 400\;nm$. Visible light emitted or reflected from objects around us provides us information about the world. Our eyes are sensitive to this range of wavelengths. Different animals are sensitive to different range of wavelengths. For example, snakes can detect infrared waves, and the ‘visible’ range of many insects extends well into the ultraviolet.
2.8.5 Ultraviolet rays
It covers wavelengths ranging from about $4 \times {10^{ - 7}}\;m\;(400\;m)$ down to $6 \times {10^{ - 10}}\;m\;(0.6\;nm)$. Ultraviolet (UV) radiation is produced by special lamps and very hot bodies. The sun is an important source of ultraviolet light. But fortunately, most of it is absorbed in the ozone layer in the atmosphere at an altitude of about $40 - 50\;km$. UV light in large quantities has harmful effects on humans. Exposure to UV radiation induces the production of more melanin, causing tanning of the skin. UV radiation is absorbed by ordinary glass. Hence, one cannot get tanned or sunburn through glass windows.
Welders wear special glass goggles or face masks with glass windows to protect their eyes from large amount of UV produced by welding arcs. Due to its shorter wavelengths, UV radiations can be focussed into very narrow beams for high precision applications such as LASIK (Laser- assisted in situ keratomileusis) eye surgery. UV lamps are used to kill germs in water purifiers.
Ozone layer in the atmosphere plays a protective role, and hence its depletion by chlorofluorocarbons (CFCs) gas (such as freon) is a matter of international concern.
2.8.6 X-rays
Beyond the UV region of the electromagnetic spectrum lies the X-ray region. We are familiar with X-rays because of its medical applications. It covers wavelengths from about ${10^{ - 8}}\;m\;(10\;nm)$ down to ${10^{ - 13}}\;m\;({10^{ - 4}}\;nm)$.
One common way to generate X-rays is to bombard a metal target by high energy electrons. X-rays are used as a diagnostic tool in medicine and as a treatment for certain forms of cancer.
Because X-rays damage or destroy living tissues and organisms, care must be taken to avoid unnecessarily or over exposure.
2.8.7 Gamma rays
They lie in the upper-frequency range of the electromagnetic spectrum and have wavelengths of from about ${10^{ - 10}}m$ to less than ${10^{ - 14}}m$.
This high-frequency radiation is produced in nuclear reactions and also emitted by radioactive nuclei. They are used in medicine to destroy cancer cells.
Summary of the different types of electromagnetic waves with their wavelength range, their source of production and detection areas shown in the table.
| Type | Wavelength range | Production | Detection |
| Radio waves | $$ > 0.1\;m$$ | Rapid acceleration and decelerations of electrons in aerials | Receiver's aerials |
| Microwaves | $$0.1\;m{\text{ to }}1\;mm$$ | Kylstron valve or magnetron valve | Point contact diodes |
| Infrared | $$1\;mm{\text{ to }}700\;mm$$ | Vibration of atoms and molecules | Thermopiles, Bolometer, Infrared photographic film |
| Visible light | $$700\;mm{\text{ to }}400\;mm$$ | Electrons in atoms emit light when they move from one energy level to a lower energy level | The eye, Photocells, Photographic film |
| Ultraviolet | $$400\;mm{\text{ to }}1\;nm$$ | Inner shell electrons in atoms moving from one energy level to lower level | Photocells, Photographic film |
| X-rays | $$1\;nm{\text{ to }}10^{-3}\;nm$$ | X-ray tubes or inner shell electrons | Photographic film, Geiger tubes, Ionization chamber |
| Gamma rays | $$ < 10^{-3}\;nm$$ | Radioactive decay of the nucleus | Photographic film, Ionization chamber, Geiger tubes |
