Electromagnetic Spectrum and its types!

  

Electric Charges

       We live in a place of visible and invisible rays of radiation. These radiations are originated from sources that possess an electromagnetic field. An electromagnetic field is a combination of both electric and magnetic fields. Every particle has a charge which makes it to have an electric field around it. This electric field is produced from charged particles like protons and electrons. This electric field surrounds the charged particle as long as the particle possesses the charge. The magnetic field is produced when the charged particles are in motion, such that all the charged particle points in the same direction and constitute the magnetic field. A moving charged particle also creates an electric field in combination with the magnetic field, thus producing together as an electromagnetic field. This process can be simply demonstrated by passing electricity through a conductor. When passing electricity, the electrons in the conductor will be in motion, which creates an electromagnetic field around it. When we place a compass near the conductor, we can see the deflection in the needle.

Credit (Electromagnetic Radiation): By And1mu - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49759107

During certain conditions, due to severe agitation by extreme temperature and several other factors, the charged particles get accelerated and disrupted, which causes a disturbance in the electromagnetic field and emits it as electromagnetic radiation or waves (moving electromagnetic field). These waves do not require any medium to travel. It travels even through a vacuum, at the speed of light. The entire universe we see now is filled with electromagnetic waves (EM waves). Electromagnetic waves travel in various frequencies depending upon the acceleration of the charged particle, and each has its own distinguishing characteristics. These waves make up the electromagnetic spectrum we know. Now, let’s briefly classify the different waves in the EM spectrum.

• Radio waves
• Microwaves
• Infrared Waves
• Visible rays
• Ultra Violet rays
• X rays
• Gamma rays

Credit(Electromagnetic Spectrum): http://en.wikipedia.org/wiki/File:EM_Spectrum_Properties_edit.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27501155

    All these waves have certain characteristics which are characterized by their frequency. A frequency is a parameter that denotes the number of cycles per second. A wave generally consists of a crest and a trough as shown in the below figure. The combination of the crest and the trough is called a cycle. If a single cycle crosses a given point at one second, it is said to be one frequency, which is denoted as the unit hertz (Hz). If 100 cycles pass that point in one second, it is said to be 100 Hz. The electromagnetic waves vary from single hertz to several exahertz (10^18). Depending upon the frequency, some of the waves could be very dangerous, and some of them will not be.

Characteristic of a wave

A wave with more frequency like gamma rays has more energy and could be very harmful to us, while the wave with lesser frequency has lesser energy, and won’t have much impact on us. This intensity can also be termed as ionizing or non-ionizing radiation. Ionizing radiation could bring a change or alter the structure; if it strikes an object. For example, if we are exposed to more amount of gamma radiation, we could have cancer; possibly death. But if we are encountered with non-ionizing radiation such as radio waves, it won’t have much impact on us. This is because of the density of the object and the energy, the wave carries. These properties are characterized by their propagation characteristics like transmission, absorption, reflection, refraction, diffraction, and scattering properties. When some waves interact with the matter, it makes it sometimes to pass through the object without disturbing the atom. This property can be seen when passing the visible light through the glass. Meanwhile, if infrared waves are made to pass through a paper, they may strike the paper molecules and make it to burn.

Difference between Ionizing and Non-Ionizing Radiation

Some of the EM waves whip around us all the time produced by various sources, like visible light from the TV, radio waves from the station, and infrared light from the furnace. One of the main sources that produce all these EM radiations is the Sun. These waves constantly collide with the earth’s atmosphere and could be fatal if we are constantly bombarded with these waves. Thus the earth’s atmosphere makes some of the waves to be allowed and some to be blocked. Particularly, high-energy waves like gamma rays, X-rays, and a part of Ultraviolet rays are blocked by the earth’s atmosphere. Here is a simple example of the penetration of different EM waves in the Earth's atmosphere.

The penetration level of different waves over Earth's atmosphere

Now let’s look at the characteristics and applications of each type of wave in the EM spectrum and how it is used by us.

Radio waves: (Frequency range: >3 kHz to 300 GHz)

Radio waves are one of the oldest and the longest wave. Radio waves are caused by various astronomical objects which leads us to the study of various astronomical events. The wavelength of the radio waves ranges from small meters to several kilometers. Radio waves were first discovered by the German physicist Heinrich Rudolph Hertz in the late 1880s. He found it by producing a spark at one antenna; which produced an electromagnetic radiation and produced a spark at the spark gap of another antenna. Many scientists began to use the power of radio waves to send signals to outer space, to find the presence of aliens. The first radio signal sent by the scientist Marconi in 1901 has traveled several trillion kilometers to date now. It has passed several stars on the way through it.

Radio waves are broadly used in communication satellites, wireless computer networks, radars, broadcasting, etc. It is also used in astronomy, for finding the greatest astronomical objects like pulsars and quasars. Due to their longer wavelength, radio wave comes under the classification of non-ionizing radiation. These waves can be easily generated due to their very low frequency. By simply supplying a varying current through a dipole antenna, the current flowing through the metal is also varied. As the charges in the metal body are oscillating, the electromagnetic field is converted into electromagnetic waves and is transmitted through space. This process can be reversed again by collecting the signals at another antenna and producing it as a time-varying supply.

Dipole antenna

 Giant antennas are placed on the ground to collect these radio signals from outer celestial objects. The astronomical objects that emit these radio signals are the sun, planets, supernova remnants, and galactic nucleus. Unlike other waves, which tend to be observed mostly, radio waves tend to be reflected, scattered, or diffracted. Due to this tendency, it is mainly used for communication purposes. The communication is mainly propagated at a straight line (line of sight), sky waves (ionosphere reflection), and ground waves (diffraction). Most of the frequency bands of radio waves can be propagated as sky waves and ground waves. As the frequency increases, the sky wave and the diffraction propagation decreases.

Radio Antenna

    The various frequency bands of radio waves and their applications are,

Extreme low frequency (>3Hz to 30Hz) - Communication between submarines, and big antennas.

Super low frequency (30 to 300Hz) - AC Power grids.

Ultra Low frequency (300to 3kz) - In mines as it can penetrate the earth.

Very low frequency (3 kHz to 30 kHz) - Military and submarine communications

Low frequency (30 kHz to 300 kHz) - Radio clocks and military communications

Medium frequency (300 kHz to 3MHz) - AM Broadcast

High frequency (3MHz to 30MHz) - Over-the-horizon radar systems, Global Maritime Distress and Safety System, aviation communications.

Very high frequency (30MHz to 300MHz) - FM, television, marine, air traffic

Microwaves: (Frequency range: 300 MHz to 300 GHz)

Microwaves are the types of waves which has a higher frequency than radio waves. It was founded by Percy Spencer and Robert N. Hall in 1945. They discovered it accidentally, by finding a chocolate bar melting in their pocket, through the radiation generated from a vacuum tube. Due to the higher frequency of the microwave, it can be only used in the propagation at the line of sight, which means, these waves do not get reflected or scattered like radio waves. Thus it is only used till the visible point. As microwaves are absorbed by the ionosphere in the atmosphere, sky wave propagation is also limited to microwaves.  But it can scatter up through the troposphere.

Communication with Microwaves

Microwaves are classified into many types, such as L band, S band, C band, X band, K_u band, K band, K_a band, Q band, U band, V band, W band, F band, and D band. Each band of microwaves has different sets of frequency ranges and is used for different applications. The main applications of microwaves are radars, wireless networks, remote sensing, keyless entry systems, and collision avoidance systems. Microwaves are generated by materials like magnetron, klystron, traveling wave tube, and gyrotron. The material magnetron is used in the microwave oven, to cook food. As the antenna size is considerably small in microwave frequency, it can be used for communication in WI-FI (802.11), walkie-talkies, Bluetooth, mobile phones, and other wireless devices. One of the major sources of the microwave is space. Our space emits a constant microwave frequency called as Cosmic Microwave Background (CMB). It is used for the study of the origin of the universe.

Credit(Cosmic Microwave Background): By European Space Agency - https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=108189337

Infrared waves (Frequency range: 300GHz to 430THz)

Infrared waves are the types of waves that are sensed by us in the form of heat. This radiation is discovered by Sir William Herschel in the year 1800 when he was measuring the temperature of the colors. When he placed a thermometer on different colors of the visible spectrum he found a higher temperature beyond the red light, which he named as calorific light; which is later derived as infrared waves. The heat we are experiencing from fire and the sun are infrared waves. This infrared radiation is used in determining the earth’s radiation through infrared absorption, reflection, and emission by the earth’s surface. Even though infrared radiation is a form of a heat wave, every other radiation in the electromagnetic spectrum has a specific amount of temperature in it. But most of the thermal radiations we experience are infrared waves. 

Credit (A girl in Infrared Radiation): By Cody.pope, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3484188

A good resistive one to infrared radiation is aluminum foil. It reflects 95% of the infrared radiation. That’s why aluminum foil is used for cooking, to have uniform heating all over the food. Infrared radiations are also used in Night vision, thermal imaging, hyperspectral imaging, optical communications, weather systems, etc. Infrared radiation is classified into three types, namely, Near Infrared, Mid Infrared, and Far Infrared. Each has its own set of frequency ranges and temperatures.

Credit(IR Radiation from a remote): By RockMancuso at the English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=15164962

Visible rays (Frequency range: 400 THz-790 THz)

As the name suggests, it is the only frequency range in the electromagnetic spectrum that is visible to the human eye. This ray is discovered by Sir Isaac Newton in the year 1666. He found it by placing a prism over the sunlight. The prism separated the sunlight into seven visible colors. That’s why he named it as visible rays. We can also see some other colors that we don’t see in the spectrum like pink, magenta, etc,. Those colors are called unsaturated colors. These colors are formed by the mixing of wavelengths. All the colors we see, even the light coming out of the device that you are currently looking at, are visible rays. Similar to other waves, which have various classifications in their frequency range, the visible rays also have their classifications. It is termed as VIBGYOR. These seven wavelengths are the colors that we see in the world. 

A Prism separating colors

Visible lights are used in astronomy for measuring the temperature of objects. If the objects glow in Orange or red color, it has low energy (For example our Sun). Similarly, if the object glows in blue color, it has high energy (For example the star Rigel).  It is also used for performing various medical procedures like laser surgery. Other common applications are LED, LCD, etc.

LED Strip Light

Ultraviolet rays (Frequency range: 750 Thz- 30PHz)

UV lamp

    Ultraviolet rays are one of the popular rays next to visible rays. It was termed by Johann Wilhelm Ritter in 1801. He discovered it by seeing a film of paper darkening beyond the violet spectrum so quickly than in other wavelengths. This light is invisible to the human eye, but it is visible to birds, animals, and insects. One of the popular examples is the house fly. That’s why to eliminate houseflies in food places, UV lights are placed with an electric cover. These houseflies get attracted to the light and get electrocuted. Even though UV light is invisible to the human eye, certain materials are mixed with the objects, to make the UV light visible. One of the popular examples is the UV lamp. This lamp is coated with phosphorous which glows when got stroked with UV light. So we can see UV light only when it is reflected, similar to visible light. Thus by showing the UV light to phosphorous-containing materials, those materials will glow. Such a method can also be used to find crime scenes and forensic analysis. The various frequency ranges of UV rays are UVA, UVB, and UVC. Let’s see the applications of these rays.

UVA (315-400nm) - for fluorescent purposes (crime scenes, bank notes printing)

UVB (280-315nm) - Health care

UVC (100-280nm) - for killing germs (purification)

Ionizing radiation starts at the UVC spectrum. As one of the major sources of UVC light is our SUN, it is blocked by the earth’s atmosphere. But our atmosphere allows both the UVA and UVB as it has less impact. But exposure to too much of them may cause sunburns and skin cancer. As UVC has a high amount of energy, it is artificially generated and is used for purification systems. (For example UV filter)

X- rays (30Petahertz- 30exahertz)

On 1895, WC Rontgen accidentally discovered these X-rays when working on an experiment. As he doesn’t know what these rays are, he named it has X-rays. These rays sit between the Ultraviolet and the Gamma rays and it is penetrable through most objects. X-rays are useful in many fields. The first thing that comes to our mind when we hear X-ray is medical. Every time, our doctor says to take an X-ray of our body parts to see a detailed image. Due to its lesser wavelength, it can pass through various objects. But these waves are stopped by high dense objects like lead, water, and minerals like calcium. That’s why we can see bones in the scan. X-rays are also used in luggage scanners in airports and stations. It is used in astronomy to study about various objects. These X-rays are classified as soft and hard X-rays. Soft X-rays have lesser energy than hard X-rays. Soft X-rays are used in X-ray microscopic imaging while hard X-rays are used in medical imaging and airport security.

X-ray picture of human hand

Symbol of Radioactive

Gamma rays (above 30exahertz)

    Gamma rays are one of the deadliest and most harmful rays in the electromagnetic spectrum. It has a higher frequency range than any other. It was discovered by chemist Paul Villard in 1900 when observing the radiation from the radium atom. The gamma rays are mainly found in radioactive elements (Unstable elements). These rays have very energy, which can strike the atoms and can alter the chemical structure. The main difference between the gamma rays from other waves is, that gamma rays are emitted from the atomic nuclei while other waves are emitted from the atomic electrons. We usually hear three basic types of particles when we hear the word radioactive elements. They are alpha, beta, and gamma. Alpha is a positively charged particle that has some amount of mass. Whereas beta is negatively charged particles, which too have some amount of mass. But gamma is a mass-less particle and it is classified as a high-energy photon. All these three elements are produced from an excited atom. The atom gets back to neutral by emitting these particles. When the gamma ray interacts with the matter it leads to,

Crompton scattering- electron out, a photon at different energy out

Photoelectric effect- electron knocked out

Coherent scattering- photon at different energy out; excitation.

Interaction of Matter

    

Interaction of waves

    Thus these three effects make the elements radioactive. Radioactive elements emitting such radiations can neutralize at different time periods. It may take from mere seconds to several billion years. One of the popular examples of a radioactive subject is the Chernobyl nuclear accident in 1986. This accident has made that place a serious radioactive place making it an unsuitable place for human life. Gamma rays have very short wavelengths that it can penetrate through any object, even the human body. It can be stopped or slowed down by lead material, concrete, etc. Most astronomical objects emit gamma rays leading to astronomical studies. Particularly when stars fall apart by black holes, it leads to the bursting of gamma rays. Even pulsars, magnetars, and solar flares emit these gamma rays. But due to its higher frequency, it is blocked from entering the earth’s atmosphere. These gamma rays are artificially produced by several materials and these materials are called radioactive elements. They are cobalt-60, caesium-137, technetium-99m and americium-241, etc. These rays are very useful in radiation therapy, sterilization in the nuclear industry, and many more.