Electromagnetic waves and light.

Electromagnetic waves and light.

1  Basic nature of e.m. waves

Electromagnetic waves are transverse waves, made of an electric field and a magnetic field that oscillate in perpendicular direction to each other.
The existence of em waves was predicted By James Clerk Maxwell in 1865. He deduced their existence rewriting the fundamental aspects of electricity and magnetism in terms of mere field.
Electric currents produce magnetic fields. Electric currents are made of moving charged particles, hence we can states that a moving charged particle produces a magnetic field. If the charged particle is at rest it produces only a static electric field in the surrounding space; if the particle moves it produces not only a varying electric field but a magnetic field too. In the field language, this statement assumes a more general and surprising form:
A VARYING ELECTRIC FIELD " E " PRODUCES A VARYING MAGNETIC FIELD " B " .

On the other hand Maxwell knews the Faraday's law: a changing magnetic field genarates a voltage in a circuit whose plane is perpendicular to the magnetic field lines, and a voltage implies an electric field, hence, even in the absence of an electric circuit he concluded that:
A VARYING MAGNETIC FIELD " B " PRODUCES A VARYING ELECTRIC FIELD " E " .
Maxwell saw this symmetry and realized that a wave involving these fields could propagated through space. [See the figure below]



A changing magnetic field produces a changing electric field, wich in turns, produces a changing magnetic field and so on. These two fields are perpendicular to each other and they are also perpendicular to the direction of travel,. Electromagnetic waves are therefore transverse waves. The process doesn't need any medium to take place, and an electromagnetic wave can propagate in a vacuum too.

2  Velocity of electromagnetic waves in a vacuum

From his mathematical equations, Maxwell could also predict the velocity of the em waves.
The velocity c of an em wave travelling in a vacuum should be equal to the square root of ratio between the Coulomb's electric constant k and the Ampere's magnetic constant k' ( k' is the constant in Ampere's expression for the force between two current carrying wires)
c=   æ
Ö
 k
k¢
 
reminding that k=9×109 [(Nm2)/(C2)] and that k¢=10-7 [ N/(A2)]
the previous formula leads to
c=   æ
Ö
(9×109  Nm2
C2
 )/(10-7  N
A2
 )
 
 =
Ö
 
9×1016
 
  m
s
 =3×108  m
s
Besides, the value c=3×108[ m/s] corrisponded with the known value of the velocity of light computed by Fizeau in 1849 (for a description of the Fizeau experiment about the velocity of light see the text on page 307). This coincidence led Maxwell to suggest that light itself is a form of electromagnetic waves . Light travels in a vacuum with the velocity "c'"and, as the other em waves, travels more slowly in the other media like air, glass, water. Anyway its velocity in air is very close to its velocity in a vacuum.

3  Spectrum of the electromagnetic radiation

Further studies will demonstrate the existence of many types of electromagnetic radiations, like radio waves, infrared waves, visible light waves, ultraviolet rays, X rays, Gamma rays. The main difference between them lies in their wavelength and frequencies. Frequencies are related to wavelength by the relationship
c=f·l       where c=3×108  m
s
By reversing the previous formula, it's possible to compute the wavelength of each em wave if the frequency is known and viceversa [See the sample exercise on page 308 in the text book].
The figure above shows the different kind of waves making up the electromagnetic spectrum. The waves differ in wavelength, frequency and in how they are generated and what materials they are able to pass through. For instance X rays will pass through materials that are opaque to visible light, Radio waves pass through walls that light cannot penetrate.



Visible light ranges only a small part of the em spectrum. The wavelength of waves is associated with colour and changes from red (approximately 700 nanometers) to violet (approximately 400 nanometers) passing progressively from orange, yellow, green and blue. At the ends of the visible region of the spectrum we have the infrared light, with longer wavelength than the red light and the ultraviolet light with smaller wavelength than the violet.



Human eyes receive and distinguish all the wavelengths of the em radiation of the visible spectrum; each wavelength is seen as light of different colour. The superposition of waves with different wavelengths, that means different colour, gives rise to different shade and tones of light. Thus, for instance, why the sun light is white can be explained: the sun radiates all the visible wavelengths at the same time and their superposition makes the light of a white tone.
See and combine different tones of colour with the java applet at: http://ww2.unime.it/dipart/i_fismed/wbt/ita/colors/colors_ita.htm


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On 23 Feb 2003, 23:59.