Electromagnetic Waves

Electrically charged objects attract or repel each other, just as two magnets attract or repel each other. The electric force that acts between charges has significant differences with the magnetic force that results in the interaction of magnets. But, for the topic at hand, these forces behave very much the same way in that they are both of the "action-at-a-distance" type. That is to say, these forces manifest themselves in the absence of any physical contact between the objects that interact with each other. For example, two magnets exert forces on each even while they are apart and neither is touching the other. In fact, the reason that a compass works is that its small magnetized needle rotates because of the magnetic force of the earth, as if manipulated by a ghost. Another way (model) of explaining this interaction is to state that the earth's magnetic core establishes a magnetic field everywhere in space. It is this field that affects the compass needle. This concept of "field" is useful because once we know the field of the earth, then we know how any magnet would be affected once it enters this field. In the case of the "force" concept, we would need, instead, to figure out every thing anew for each magnet. But aside from this practical use, the force and the field concepts are the same.

Another advantage of the field concept is that it allows for an easier visualization of what happens when one of the charges (or magnets) begin to move. In this view, when a charge changes position, then the field that it produces also changes in space. In fact, as the charge oscillates, so does its field. This oscillating field is what is called the electromagnetic wave. This phenomena is very much the same as the case of the model that we examined earlier involving a tube of billiard balls connected to each other by springs. By setting the ball at one end of the tube into oscillations eventually the ball at the other end started to oscillate. In the same way by making a charge oscillate at one point in space we can cause another charge located further away to undergo oscillatory motion. Similar to mechanical waves, such as sound and water waves, electromagnetic waves are characterized by their frequency, speed, and amplitude.

The above picture shows how both the magnetic and electric fields oscillate as the wave propagates to the right.

One interesting aspect of electromagnetic wave that sets it apart from all other waves we have examined so far is that its propagation requires no medium. Water waves, which are transverse, of course need water to propagate in. Sound waves, which are longitudinal, also need a matter medium; although almost any type of matter would do for them (sound travels in air, all known gases, in fluids, in solids, and in plasma). But oscillating electromagnetic fields travel even in vacuum. Another interesting feature of electromagnetic waves relates to their speed of propagation. Mechanical waves travel with a speed that is characteristic of their medium of travel. For example, sound travels much faster in metals than it does in air. Its speed of travel in air is also dependant on the air pressure, temperature, and humidity. In the same way, speed of propagation of electromagnetic waves depends on the material it is passing through, even though it does not "need" the material for its propagation. This wave travels its fastest in vacuum, with a speed of three hundred million meters per second (300,000,000 m/s or 3x10⁸ m/s). But what is most significant is that this speed, to our knowledge, is the fastest possible way that nature allows for transmission of energy and momentum.

A third feature of electromagnetic waves that separates it from mechanical waves is that the energy that it propagates also depends on its frequency (of oscillation). This feature is related to quantization phenomena that we will study in a future topic. As we will examine later, experimental evidence shows that electromagnetic wave can be represented as if it were made of fixed chunks, or quanta, of traveling oscillations, called photons. These photons all travel with the same speed

                    photon speed = c = 300,000,000 m/s or 3x10⁸ m/s    (in vacuum),

but each individual photon's energy depends on its frequency, n (Greek letter, called nu). To be exact, the photon energy, Eg, is just the product of its frequency value, f, measured in Hertz, with a constant that is named after the German Physicist who came up with this idea, Planck. Planck's constant, denoted by the letter h has a value of 6.63x10^-34 Joule.second; 

    i.e.     photon energy in Joules:       Eg = h  f= (6.63x10^-34) (value of photon's frequency in Hz).   

The amplitude of the electromagnetic wave, in this photon picture, is a measure of the number of photons that are traveling together to make up the wave. So, again, amplitude relates to the energy of propagation. 

Electromagnetic wave, photons, interact with charged entities. Atoms, molecules, and solids all have charges that varies in quality and quantity among them. As a result, electromagnetic waves of different frequency interact differently with the medium through which they propagate. For purely practical reasons, then, we classify ranges of these frequencies as "bands" and give them specific names. For example, very high frequency photons (those with frequencies larger than 10²⁰ Hz) are called gamma-rays. These photons tend to only interact with the atomic nuclei, which are extremely compact in structure. So, gamma-rays pass through even very thick concrete walls. X-rays are photons belonging to a band of frequencies any where from 10¹⁶ to 10²⁰ Hz. They can go though material objects too, but they do not have as large a penetration range as the gamma-rays. For example, X-rays of the frequency used in medicine penetrate through most tissue, except the denser ones in bones or tumors. These ranges of frequencies is referred to as the electromagnetic spectrum. Visible light is just a very narrow band of this spectrum. Its frequency is of the order of 10¹⁴ Hz. Our visual system happens to respond to this frequency photons by generating electric pulses in the visual nervous system, which we is interpreted by our brain as sight.

 Electromagnetic Spectrum

Spectrum of Electromagnetic Radiation
Regions	Wavelength (Angstroms)	Wavelength (meters)	Frequency (Hz)	Energy (eV)
Radio	greater than 109	greater than 1	less than 3 x 109	less than 10-5
Microwave	109 - 106	0.1 - 0.01	3 x 109 - 3 x 1012	10-5 - 0.01
Infrared	106 - 7000	0.0001 - 7 x 10-7	3 x 1012 - 4.3 x 1014	0.01 - 2
Visible	7000 - 4000	7 x 10-7 - 4 x 10-7	4.3 x 1014 - 7.5 x 1014	2 - 3
Ultraviolet	4000 - 10	4 x 10-7 - 10-9	7.5 x 1014 - 3 x 1017	3 - 103
X-Rays	10 - 0.1	10-9 - 10-11	3 x 1017 - 3 x 1019	103 - 105
Gamma-Rays	less than 0.1	less than 10-11	greater than 3 x 1019	greater than 105

These values are very approximate and only denote a range. eV is a unit of energy. It is the energy that an electron acquires once it passes through a 1 volt potential difference. It is useful for study of atoms. Note the energy for the visible section.

Last Modified: September 10, 2007 malekis@union.edu