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Question:
How come light waves and sound waves travel in different types of waves? I know they do but why?



Replies:
Well, they are different in many ways! Sound is caused by pressure changes in the air, so sound waves are "pressure" waves. Because pressure is just a number (actually in a solid it is a second-rank tensor but we do not need to get into that) the wave just consists of this number going up and down along one wavelength, and travelling at the speed of sound. Light (and other Electro-Magnetic radiation) is caused by oscillations of electric and magnetic fields. These fields are not just numbers, but have a direction to them (the direction in which a charged particle would move under the force of the electric field gives its direction) - they are vectors. Since a direction can have 3 cartesian components (we are living in 3-dimensional space) that means there can be 3 different components of light. Actually, one of those components will lie in the direction of propagation and is associated with the motion of charge in a medium - so called plasma waves - which does not apply to light in a vacuum or in the air. So that leaves the two different allowed directions for the electric fields, giving the two polarization you can observe with polarizes etc. Light just consists of these fields changing their direction and magnitude with a period in space of the wavelength of the light, and a frequency equal to the light frequency, travelling at the speed of light (very fast!)

Arthur Smith



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