PAPER - 604 : ELECTROMAGNETIC RADIATION (EMR)

Electromagnetic Radiation (EMR)

Electromagnetic radiation is a form of energy that is produced by oscillating electric and magnetic disturbance, or by the movement of electrically charged particles traveling through a vacuum or matter. The electric and magnetic fields come at right angles to each other and combined wave moves perpendicular to both magnetic and electric oscillating fields thus the disturbance. Electron radiation is released as photons, which are bundles of light energy that travel at the speed of light as quantized harmonic waves. This energy is then grouped into categories based on its wavelength into the electromagnetic spectrum. These electric and magnetic waves travel perpendicular to each other and have certain characteristics, including amplitude, wavelength, and frequency.

Electromagnetic (EM) radiation is a form of energy that is all around us and takes many forms, such as radio waves, microwaves, X-rays and gamma rays. Sunlight is also a form of EM energy, but visible light is only a small portion of the EM spectrum, which contains a broad range of electromagnetic wavelengths.

General Properties of all electromagnetic radiation:

Electromagnetic radiation can travel through empty space. Most other types of waves must travel through some sort of substance. For example, sound waves need either a gas, solid, or liquid to pass through in order to be heard.

The speed of light is always a constant. (Speed of light : 2.99792458 x 10⁸ m s^-1)

Wavelengths are measured between the distances of either crests or troughs. It is usually characterized by the Greek symbol .

Electromagnetic Theory:

Electricity and magnetism were once thought to be separate forces. However, in 1873, Scottish physicist James Clerk Maxwell developed a unified theory of electromagnetism. The study of electromagnetism deals with how electrically charged particles interact with each other and with magnetic fields.

There are four main electromagnetic interactions:

The force of attraction or repulsion between electric charges is inversely proportional to the square of the distance between them.

Magnetic poles come in pairs that attract and repel each other, much as electric charges do.

An electric current in a wire produces a magnetic field whose direction depends on the direction of the current.

A moving electric field produces a magnetic field, and vice versa.

Maxwell also developed a set of formulas, called Maxwell's equations, to describe these phenomena.

Waves and fields:

EM radiation is created when an atomic particle, such as an electron, is accelerated by an electric field, causing it to move. The movement produces oscillating electric and magnetic fields, which travel at right angles to each other in a bundle of light energy called a photon. Photons travel in harmonic waves at the fastest speed possible in the universe: 186,282 miles per second (299,792,458 meters per second) in a vacuum, also known as the speed of light. The waves have certain characteristics, given as frequency, wavelength or energy.

Electromagnetic waves are formed when an electric field (shown in red arrows) couples with a magnetic field (shown in blue arrows). Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave. (Image credit: NOAA.)

Waves and their Characteristics:

                                   

 Electromagnetic Waves

An EM Wave

Amplitude

Amplitude is the distance from the maximum vertical displacement of the wave to the middle of the wave. This measures the magnitude of oscillation of a particular wave. In short, the amplitude is basically the height of the wave. Larger amplitude means higher energy and lower amplitude means lower energy. Amplitude is important because it tells you the intensity or brightness of a wave in comparison with other waves.

Wavelength

A wavelength is the distance between two consecutive peaks of a wave. Wavelength (λ) is the distance of one full cycle of the oscillation.This distance is given in meters (m) or fractions thereof. Frequency is the number of waves that form in a given length of time. It is usually measured as the number of wave cycles per second, or hertz (Hz).

A short wavelength means that the frequency will be higher because one cycle can pass in a shorter amount of time. Similarly, a longer wavelength has a lower frequency because each cycle takes longer to complete. 

Longer wavelength waves such as radio waves carry low energy; this is why we can listen to the radio without any harmful consequences. Shorter wavelength waves such as x-rays carry higher energy that can be hazardous to our health. Consequently lead aprons are worn to protect our bodies from harmful radiation when we undergo x-rays. This wavelength frequently relationship is characterized by:

c=λν(1)

where

c is the speed of light,

λ is wavelength, and

ν is frequency.

Shorter wavelength means greater frequency, and greater frequency means higher energy. Wavelengths are important in that they tell one what type of wave one is dealing with.

                                      Different Wavelengths and Frequencies

Remember, Wavelength tells you the type of light and Amplitude tells you about the intensity of the light

Frequency:

Frequency is defined as the number of cycles per second, and is expressed as sec^-1 or Hertz (Hz). Frequency is directly proportional to energy and can be express as:

E=hν(2)

where

E is energy,

h is Planck's constant, (h= 6.62607 x 10^-34 J), and

ν

is frequency.

Period:

Period (T) is the amount of time a wave takes to travel one wavelength; it is measured in seconds (s).

Velocity:

The velocity of wave in general is expressed as:

velocity=λν(3)

For Electromagnetic wave, the velocity in vacuum is 2.99×108m/s

or 186,282 miles/second.

The EM spectrum:

EM radiation spans an enormous range of wavelengths and frequencies. This range is known as the electromagnetic spectrum. The EM spectrum is generally divided into seven regions, in order of decreasing wavelength and increasing energy and frequency. The common designations are: radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma rays. Typically, lower-energy radiation, such as radio waves, is expressed as frequency; microwaves, infrared, visible and UV light are usually expressed as wavelength; and higher-energy radiation, such as X-rays and gamma rays, is expressed in terms of energy per photon. 

The electromagnetic spectrum is generally divided into seven regions, in order of decreasing wavelength and increasing energy and frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. (Image credit: Biro Emoke Shutterstock)

Radiation Types

Radio waves:

Radio waves are at the lowest range of the EM spectrum, with frequencies of up to about 30 billion hertz, or 30 gigahertz (GHz), and wavelengths greater than about 10 millimeters (0.4 inches).

Radio Waves are approximately 10³ m in wavelength. As the name implies, radio waves are transmitted by radio broadcasts, TV broadcasts, and even cell phones. Radio waves have the lowest energy levels. Radio waves are used in remote sensing, where hydrogen gas in space releases radio energy with a low frequency and is collected as radio waves. They are also used in radar systems, where they release radio energy and collect the bounced energy back. Especially useful in weather, radar systems are used to can illustrate maps of the surface of the Earth and predict weather patterns since radio energy easily breaks through the atmosphere. ;

Radio is used primarily for communications including voice, data and entertainment media.

Microwaves:

Microwaves fall in the range of the EM spectrum between radio and IR. They have frequencies from about 3 GHz up to about 30 trillion hertz, or 30 terahertz (THz), and wavelengths of about 10 mm (0.4 inches) to 100 micrometers (μm), or 0.004 inches.

Microwaves can be used to broadcast information through space, as well as warm food. Microwaves are used for high-bandwidth communications, radar and as a heat source for microwave ovens and industrial applications. They are also used in remote sensing in which microwaves are released and bounced back to collect information on their reflections.

Microwaves can be measured in centimeters. They are good for transmitting information because the energy can go through substances such as clouds and light rain. Short microwaves are sometimes used in Doppler radars to predict weather forecasts.

Infrared:

Infrared is in the range of the EM spectrum between microwaves and visible light. IR has frequencies from about 30 THz up to about 400 THz and wavelengths of about 100 μm (0.004 inches) to 740 nanometers (nm), or 0.00003 inches. IR light is invisible to human eyes, but we can feel it as heat if the intensity is sufficient. 

Infrared radiation can be released as heat or thermal energy. It can also be bounced back, which is called near infrared because of its similarities with visible light energy. Infrared Radiation is most commonly used in remote sensing as infrared sensors collect thermal energy, providing us with weather conditions.

                     This picture represents a snap shot in mid-infrared light.

Visible light:

Visible Light is the only part of the electromagnetic spectrum that humans can see with an unaided eye. This part of the spectrum includes a range of different colors that all represent a particular wavelength. Rainbows are formed in this way; light passes through matter in which it is absorbed or reflected based on its wavelength. Thus, some colors are reflected more than other, leading to the creation of a rainbow.

Visible light is found in the middle of the EM spectrum, between IR and UV. It has frequencies of about 400 THz to 800 THz and wavelengths of about 740 nm (0.00003 inches) to 380 nm (.000015 inches). More generally, visible light is defined as the wavelengths that are visible to most human eyes.

 

Ultraviolet:

Ultraviolet light is in the range of the EM spectrum between visible light and X-rays. It has frequencies of about 8 × 10¹⁴ to 3 × 10¹⁶ Hz and wavelengths of about 380 nm (.000015 inches) to about 10 nm (0.0000004 inches). UV light is a component of sunlight; however, it is invisible to the human eye. It has numerous medical and industrial applications, but it can damage living tissue. UV radiation is most commonly known because of its severe effects on the skin from the sun, leading to cancer.

X-Rays:

X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays comprise the range of the EM spectrum between UV and gamma rays. Soft X-rays have frequencies of about 3 × 10¹⁶ to about 10¹⁸ Hz and wavelengths of about 10 nm (4 × 10⁻⁷ inches) to about 100 picometers (pm), or 4 × 10⁻⁸ inches. Hard X-rays occupy the same region of the EM spectrum as gamma rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma rays are produced by atomic nuclei.

X-rays are used to produce medical images of the body.

Gamma-Rays:

Gamma-rays are in the range of the spectrum above soft X-rays. Gamma-rays have frequencies greater than about 10¹⁸ Hz and wavelengths of less than 100 pm (4 × 10⁻⁹ inches). Gamma radiation causes damage to living tissue, which makes it useful for killing cancer cells when applied in carefully measured doses to small regions. Uncontrolled exposure, though, is extremely dangerous to humans.

Gamma Rays can used in chemotherapy in order to rid of tumors in a body since it has such a high energy level. The shortest waves, Gamma rays, are approximately 10^-12 m in wavelength. Out this huge spectrum, the human eyes can only detect waves from 390 nm to 780 nm.

Interference