The Electromagnetic Spectrum by Year 12 Physics Students

Electromagnetic Waves

by Milan Divecha 12Y

What are electromagnetic waves?

Electromagnetic waves are waves that are a self-propagating composite of an electric field and a magnetic field. They transport energy stored in the propagated electric and magnetic fields. (1) The magnetic and electric fields are always perpendicular to each other and they are always in phase with each other. (1) The propagation of the wave is perpendicular to both the electric and magnetic fields (see diagram below). (3)

How are they caused?

To understand the cause of electromagnetic waves further, consider Faraday’s law of induction. The law of electromagnetic induction states that a moving magnetic field will induce a current in a wire, and a moving electric charge will create a magnetic field around a wire. Similarly, when an electric charge vibrates, a changing electric field is created. By applying the law of electromagnetic induction, we can see that this would cause a changing magnetic field. This would then cause a changing electric field and the process would keep repeating, so the two fields continuously create each other. Hence, the wave is “self-sustaining”. (4) This means that the energy is transferred by the two oscillating fields without a need for a medium. The initial electric charge vibration is often from a vibrating electron or from

The picture below (3) illustrates this:

image

Properties of Electromagnetic Waves:

An electromagnetic wave can be described in terms of its energy—in units of electron volts (eV). An electron volt is the amount of kinetic energy needed to move an electron through one volt potential. Moving along the spectrum from long to short wavelengths, energy increases as the wavelength shortens. (5)

All electromagnetic waves transport their energy through a vacuum at a speed of 3.00 x E8 m/s (a value commonly represented by the symbol c). The propagation of an electromagnetic wave through a material medium occurs at a speed which is less than 3.00 x E8 m/s.(2) Einstein theorised that no particle or object can travel faster than this speed (the speed of light) (6) .

image

http://en.wikipedia.org/wiki/Albert_Einstein

Albert Einstein (14 March 1879 – 18 April 1955) was a German-born theoretical physicist.

The actual speed of an electromagnetic wave through a material medium is dependent upon the density of that medium; different materials have their atoms more closely packed than others therefore the amount of distance between the atoms is less. Different materials cause a different amount of delay due to the absorption and reemission process.

Electromagnetic waves are all transverse waves, so they can all be reflected, refracted, diffracted and polarised. (4)

Electromagnetic Spectrum:

The different types of electromagnetic waves form a spectrum ordered by their wavelengths. It is important to know that the electromagnetic spectrum is continuous, meaning there is no definite cut-off points between types of waves and the regions merge together. Since they all follow the wave equation speed = frequency x wavelength (c = f x l) and the speed is constant in a vacuum moving along the spectrum the wavelength gets greater and the frequency becomes lower. (9)

image

Historical context of electromagnetic waves:

For many years, people had observed links between electricity and magnetism and physicists like Michael Faraday and Andre-Marie Ampere had created laws connecting them. Faraday, for example, wrote the law of induction, which stated that a moving magnetic field will induce a current in a wire, and a moving electric charge will create a magnetic field around a wire (due to changing magnetic and electric fields). However, it wasn’t until 1864, a Scottish physicist named James Clerk Maxwell discovered the electromagnetic wave. Furthermore, he produced a theory to explain light in terms of electric and magnetic fields, meaning he was the first to even think of light as a wave that was connected to electricity and magnetism (i.e. an electromagnetic wave). He used the ideas of Michael Faraday and Andre-Marie Ampere, who set out laws relating the movement of electric charge to changes in magnetic fields. He suggested that electromagnetic waves are caused by an oscillating electric charge which sets up an oscillating electric field. As a result, an oscillating magnetic field is caused, causing an oscillating electric field etc… thus leading to a self-sustaining electromagnetic wave (this is what was explained earlier). He was able to show using maths that the speed of the propagation of an electromagnetic wave would be the same as the speed of light, which helped to prove that light, was an electromagnetic wave. (11) Then in 1887, the German physicist Heinrich Hertz in a series of experiments proved the actual existence electromagnetic waves, showing that transverse electromagnetic waves can travel over some distance as predicted by Maxwell and Faraday. Hertz published his work in a book titled: “Electric waves: being researches on the propagation of electric action with finite velocity through space”. The discovery of electromagnetic waves in space led to the development in the closing years of the 19th century of radio. He also showed that radio waves travelled at the speed of light, which showed the two were linked and were both electromagnetic waves. (11)

image

http://en.wikipedia.org/wiki/Heinrich_Hertz

Heinrich Rudolf Hertz (22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of electromagnetic waves theorized by James Clerk Maxwell’s electromagnetic theory of light.

Benefits:

Our lives revolve heavily around electromagnetic waves. Everything we see is as a result of absorbing light, which is an electromagnetic wave. This means that any technology which involves our sight was designed with knowledge of electromagnetic waves. In addition, we make use of radio waves everyday with radios; we use X-rays and gamma rays for important medical applications. Also, all our digital communication is done using microwaves, radio waves or light (not needing a medium and having a fast speed makes electromagnetic waves good for communication and information transfer.)

Risks:

Some may argue that the increased use of electromagnetic waves over the years could be harmful to us. For example, recently people have used ultraviolet light tanning beds for artificial skin tanning. This has been linked to an increased risk in skin cancer. Increased exposure to X-rays and gamma rays has also been proven to cause cancer since their extremely short wavelengths and high frequencies (thus higher energy) cause them to be absorbed by body cells easily. This can lead to the DNA being altered which can cause harmful mutations. (7) There are also concerns that despite microwaves having longer wavelengths, they could cause brain damage in children since they have thinner skulls and they tend to use their phones more than is absolutely necessary. However, there is no proof for this and these claims have been disputed because cell phones use a very low level of radio frequency (RF) energy – too low to cause damage. Also, the wave emitted is non-ionizing – meaning it doesn’t cause damage to chemical bonds or DNA. (8)

Bibliography:

1. Internet link- http://hyperphysics.phy-astr.gsu.edu/hbase/waves/emwavecon.html

2. Internet link- http://www.physicsclassroom.com/mmedia/waves/em.cfm

3. Picture and internet link- http://micro.magnet.fsu.edu/primer/java/polarizedlight/emwave/

Last updated: Wednesday, Mar 26, 2014 at 01:24 PM

4. Textbook-Edexcel AS Physics Students’ Book By Miles Hudson and Patrick Fullick. ISBN: 978-1-4058-9638-2 . Date of publishing: 2008

5. Internet link- http://missionscience.nasa.gov/ems/02_anatomy.html

Last updated: 13/8/14

6. Book- A Brief History of Time By Stephen Hawking. ISBN: 978-0-857-50100-4. Date of Published: 1988

7. Internet link- http://www.cancer.org/cancer/cancercauses/radiationexposureandcancer/xraysgammaraysandcancerrisk/x-rays-gamma-rays-and-cancer-risk-do-xrays-and-gamma-rays-cause-cancer

Last Updated: 04/18/2014

8. Internet link- http://www.ehso.com/ehshome/cellphonecancer.php

Revised: 2014-Sep-10

9. Video- Electromagnetic waves and the electromagnetic spectrum: https://www.youtube.com/watch?v=7eutept5h0Q

Posted: 7/7/14

10. Picture- http://www.cyberphysics.co.uk/topics/light/emspect.htm

11. Internet link- http://en.wikipedia.org/wiki/History_of_electromagnetic_theory

Last updated: 24/1/15

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