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Transcript
Transversal Waves The Question This is a question about physics. I know the difference between transversal and longitudinal waves. My question relates to transversal waves. I can comprehend why a wave on a surface of water or along a string is transversal, and why sound waves are longitudinal. But why is electromagnetic radiation transversal? In other words, why does, for instance, visible light move up and down between two extremes while moving? Why does it reverse direction at the high and low points of a wave, and why does it continually change its direction? The Answer Electricity and magnetism are forces that are active even from a distance, as opposed to forces which require direct contact such as friction and pressure. If one electrically charges an object by rubbing it up, a force can be felt even at a distance, such as with a comb which attracts snippets of paper. The same goes for magnets, which can attract nails without direct contact. Because this force works from a distance, electromagnetism is described as a force field. Objects will experience this force, if they are sensitive to it. Gravity is another example of a force active at a distance, which can also be described as a force field. If a vertical livewire, connected to a battery, is stuck through a horizontal cardboard with some iron filings on top of it, the iron filings will form circles around the livewire at the centre. The filings will indicate the direction of the magnetic field. The electric current, flowing through the wire, is created by an electric field powered by the battery. The electric field will move the electrons along the wire. As shown by the filings, a magnetic field will be generated by the electric field, perpendicular to this electric field. However, the circular magnetic field will not suddenly exist everywhere. Just like ripples of water stirred by a stone thrown in a pool, the magnetic field will expand in all directions because the circles become bigger and bigger. There is an important difference, however: the direction of the magnetic field travels along the circles, whereas waves of water are perpendicular to the force. Should a livewire be connected to an alternating electric field, a magnetic field will be created expanding in circles in continually changing directions. A pattern of consecutive circles will be created along a magnetic field that is continually varying in size and direction. The magnetic field will generate an electric field itself as well. This is exactly how a dynamo works: a magnet spins around within a coil, generating an electric field. An electric current is thus powered. The electric field will be perpendicular to the magnetic field. Because of the alternating electric field in the livewire, an alternating circular electric field is created parallel to the live wire, and an alternating magnetic field perpendicular to the live wire will be created at the same time. Both fields will expand in all directions in bigger and bigger circles. This is called an electromagnetic wave. Because both the electric and magnetic field are perpendicular to the direction of the propagation, the electromagnetic wave is called transversal. Should one picture this wave at one moment in time, points in space may be distinguished where the electric field is pointing upwards, and pointing downwards a little further on. Somewhere between these points, the strength of the electric field will hence be zero. The same goes for the magnetic field. In order to picture the field strength, a line is often drawn from the source – the livewire, in this case – perpendicular to the direction and strength of the fields. This line will look like a wave, even though it is not! As shown, electromagnetic waves do not behave like waves along a string. Therefore, light does not move on between two extremes. Light just moves straight ahead. Kind regards, Drs. H. Jordens Faculty of Mathematics and Natural Sciences University of Groningen Translated by Anniversary Project Group